feat: Implement Klarna payment module

src/Smartstore.Modules/Smartstore.Klarna/Client/CreateCustomerTokenRequest.cs

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+using Newtonsoft.Json;
+
+namespace Smartstore.Klarna.Client
+{
+    public class CreateCustomerTokenRequest : KlarnaApiRequest
+    {
+        // Properties based on Klarna API for generating a customer token
+        [JsonProperty("purchase_country")]
+        public string PurchaseCountry { get; set; }
+
+        [JsonProperty("purchase_currency")]
+        public string PurchaseCurrency { get; set; }
+
+        [JsonProperty("locale")]
+        public string Locale { get; set; }
+
+        [JsonProperty("description")]
+        public string Description { get; set; }
+
+        [JsonProperty("intended_use")] // e.g. SUBSCRIPTION
+        public string IntendedUse { get; set; }
+
+        [JsonProperty("merchant_urls")]
+        public KlarnaMerchantUrls MerchantUrls { get; set; }
+    }
+}

src/Smartstore.Modules/Smartstore.Klarna/Client/CreateOrderRequest.cs

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+using Newtonsoft.Json;
+
+namespace Smartstore.Klarna.Client
+{
+    public class CreateOrderRequest : KlarnaApiRequest
+    {
+        // Properties based on Klarna API for creating an order
+        // Often similar to CreateSessionRequest but might include authorization_token
+        [JsonProperty("purchase_country")]
+        public string PurchaseCountry { get; set; }
+
+        [JsonProperty("purchase_currency")]
+        public string PurchaseCurrency { get; set; }
+
+        [JsonProperty("locale")]
+        public string Locale { get; set; }
+
+        [JsonProperty("order_amount")]
+        public long OrderAmount { get; set; }
+
+        [JsonProperty("order_tax_amount")]
+        public long OrderTaxAmount { get; set; }
+
+        [JsonProperty("order_lines")]
+        public KlarnaOrderLine[] OrderLines { get; set; }
+
+        // Billing and shipping addresses, customer details etc.
+    }
+}

src/Smartstore.Modules/Smartstore.Klarna/Client/CreateOrderResponse.cs

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+using Newtonsoft.Json;
+
+namespace Smartstore.Klarna.Client
+{
+    public class CreateOrderResponse : KlarnaApiResponse
+    {
+        // Properties based on Klarna API response for creating an order
+        // e.g., OrderId, RedirectUrl, FraudStatus etc.
+        [JsonProperty("order_id")]
+        public string OrderId { get; set; }
+
+        [JsonProperty("redirect_url")]
+        public string RedirectUrl { get; set; }
+
+        [JsonProperty("fraud_status")]
+        public string FraudStatus { get; set; }
+    }
+}

src/Smartstore.Modules/Smartstore.Klarna/Client/CreateSessionRequest.cs

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+using Newtonsoft.Json;
+
+namespace Smartstore.Klarna.Client
+{
+    public class CreateSessionRequest : KlarnaApiRequest
+    {
+        // Properties based on Klarna API for creating a session
+        // e.g., OrderAmount, OrderLines, PurchaseCountry, Currency, Locale, MerchantUrls etc.
+        [JsonProperty("purchase_country")]
+        public string PurchaseCountry { get; set; }
+
+        [JsonProperty("purchase_currency")]
+        public string PurchaseCurrency { get; set; }
+
+        [JsonProperty("locale")]
+        public string Locale { get; set; }
+
+        [JsonProperty("order_amount")]
+        public long OrderAmount { get; set; }
+
+        [JsonProperty("order_tax_amount")]
+        public long OrderTaxAmount { get; set; }
+
+        [JsonProperty("order_lines")]
+        public KlarnaOrderLine[] OrderLines { get; set; }
+
+        [JsonProperty("merchant_urls")]
+        public KlarnaMerchantUrls MerchantUrls { get; set; }
+    }
+}

src/Smartstore.Modules/Smartstore.Klarna/Client/CreateSessionResponse.cs

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+using Newtonsoft.Json;
+
+namespace Smartstore.Klarna.Client
+{
+    public class CreateSessionResponse : KlarnaApiResponse
+    {
+        // Properties based on Klarna API response for creating a session
+        // e.g., SessionId, ClientToken, PaymentMethodCategories etc.
+        [JsonProperty("session_id")]
+        public string SessionId { get; set; }
+
+        [JsonProperty("client_token")]
+        public string ClientToken { get; set; }
+    }
+}

src/Smartstore.Modules/Smartstore.Klarna/Client/CustomerTokenResponse.cs

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+using Newtonsoft.Json;
+
+namespace Smartstore.Klarna.Client
+{
+    public class CustomerTokenResponse : KlarnaApiResponse
+    {
+        [JsonProperty("token_id")]
+        public string TokenId { get; set; }
+
+        [JsonProperty("redirect_url")]
+        public string RedirectUrl { get; set; }
+    }
+}

src/Smartstore.Modules/Smartstore.Klarna/Client/KlarnaApiConfig.cs

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+namespace Smartstore.Klarna.Client
+{
+    // Placeholder for Klarna API Configuration
+    public class KlarnaApiConfig
+    {
+        public string ApiUrl { get; set; } // e.g., "https://api.klarna.com/" or "https://api.playground.klarna.com/"
+        public string ApiKey { get; set; } // Username or Merchant ID
+        public string ApiSecret { get; set; } // Password or Shared Secret
+    }
+}

src/Smartstore.Modules/Smartstore.Klarna/Client/KlarnaApiException.cs

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+using System;
+
+namespace Smartstore.Klarna.Client
+{
+    public class KlarnaApiException : Exception
+    {
+        public KlarnaApiException(string message) : base(message)
+        {
+        }
+
+        public KlarnaApiException(string message, Exception innerException) : base(message, innerException)
+        {
+        }
+    }
+}

src/Smartstore.Modules/Smartstore.Klarna/Client/KlarnaApiRequest.cs

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+namespace Smartstore.Klarna.Client
+{
+    public class KlarnaApiRequest
+    {
+        // TODO: Implement request properties
+    }
+}

src/Smartstore.Modules/Smartstore.Klarna/Client/KlarnaApiResponse.cs

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+namespace Smartstore.Klarna.Client
+{
+    public class KlarnaApiResponse
+    {
+        // TODO: Implement response properties
+    }
+}

src/Smartstore.Modules/Smartstore.Klarna/Client/KlarnaHttpClient.cs

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+using Newtonsoft.Json;
+using System;
+using System.Net.Http;
+using System.Net.Http.Headers;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Smartstore.Klarna.Client
+{
+    public class KlarnaHttpClient
+    {
+        private readonly HttpClient _httpClient;
+        private readonly KlarnaApiConfig _apiConfig; // Assuming a config class for API key, secret, and base URL
+
+        public KlarnaHttpClient(HttpClient httpClient, KlarnaApiConfig apiConfig)
+        {
+            _httpClient = httpClient ?? throw new ArgumentNullException(nameof(httpClient));
+            _apiConfig = apiConfig ?? throw new ArgumentNullException(nameof(apiConfig));
+
+            _httpClient.BaseAddress = new Uri(_apiConfig.ApiUrl); // e.g., "https://api.klarna.com/"
+            _httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
+            // Basic Authentication: username:password -> base64
+            var authToken = Convert.ToBase64String(Encoding.ASCII.GetBytes($"{_apiConfig.ApiKey}:{_apiConfig.ApiSecret}"));
+            _httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", authToken);
+        }
+
+        // Example: Create Credit Session
+        public async Task<CreateSessionResponse> CreateCreditSessionAsync(CreateSessionRequest request)
+        {
+            var requestUri = "payments/v1/sessions"; // Example endpoint
+            return await SendRequestAsync<CreateSessionResponse>(HttpMethod.Post, requestUri, request);
+        }
+
+        // Example: Update Credit Session
+        public async Task UpdateCreditSessionAsync(string sessionId, UpdateSessionRequest request)
+        {
+            var requestUri = $"payments/v1/sessions/{sessionId}"; // Example endpoint
+            await SendRequestAsync<object>(HttpMethod.Post, requestUri, request); // No specific response body for update, or define one if needed
+        }
+
+        // Example: Get Session Details
+        public async Task<SessionDetailsResponse> GetCreditSessionAsync(string sessionId)
+        {
+            var requestUri = $"payments/v1/sessions/{sessionId}"; // Example endpoint
+            return await SendRequestAsync<SessionDetailsResponse>(HttpMethod.Get, requestUri);
+        }
+
+        // Example: Create Order
+        public async Task<CreateOrderResponse> CreateOrderAsync(string authorizationToken, CreateOrderRequest request)
+        {
+            var requestUri = $"ordermanagement/v1/orders"; // Example endpoint for Klarna Order Management API
+            // For order creation, Klarna might require the authorization_token from the payment session
+            // This might need a different setup or HttpClient instance if the base URL or auth changes for OMS API
+            return await SendRequestAsync<CreateOrderResponse>(HttpMethod.Post, requestUri, request);
+        }
+
+        // Example: Generate Customer Token
+        public async Task<CustomerTokenResponse> GenerateCustomerTokenAsync(CreateCustomerTokenRequest request)
+        {
+            var requestUri = "customer-token/v1/tokens"; // Example endpoint
+            return await SendRequestAsync<CustomerTokenResponse>(HttpMethod.Post, requestUri, request);
+        }
+
+        private async Task<TResponse> SendRequestAsync<TResponse>(HttpMethod method, string requestUri, object requestData = null)
+        {
+            var requestMessage = new HttpRequestMessage(method, requestUri);
+
+            if (requestData != null)
+            {
+                var jsonRequest = JsonConvert.SerializeObject(requestData);
+                requestMessage.Content = new StringContent(jsonRequest, Encoding.UTF8, "application/json");
+            }
+
+            HttpResponseMessage responseMessage = await _httpClient.SendAsync(requestMessage);
+
+            if (!responseMessage.IsSuccessStatusCode)
+            {
+                var errorContent = await responseMessage.Content.ReadAsStringAsync();
+                // TODO: Deserialize errorContent into a KlarnaError object if Klarna provides a standard error format
+                throw new KlarnaApiException($"Klarna API request failed with status code {responseMessage.StatusCode}: {errorContent}");
+            }
+
+            var jsonResponse = await responseMessage.Content.ReadAsStringAsync();
+            if (string.IsNullOrWhiteSpace(jsonResponse) && typeof(TResponse) != typeof(object))
+            {
+                // Handle cases where Klarna might return an empty body for success (e.g., 204 No Content)
+                // and TResponse is not expecting it (e.g. not 'object' or a specific 'EmptyResponse' type)
+                if (responseMessage.StatusCode == System.Net.HttpStatusCode.NoContent)
+                {
+                    return default; // Or throw, or return a specific object indicating success with no content
+                }
+                throw new KlarnaApiException("Received an empty response from Klarna API when content was expected.");
+            }
+
+            // If TResponse is object, it means we don't expect a specific response body or it's handled by status code.
+            if (typeof(TResponse) == typeof(object))
+            {
+                return default; // Or a new object(), depending on how you want to signal this.
+            }
+
+            try
+            {
+                return JsonConvert.DeserializeObject<TResponse>(jsonResponse);
+            }
+            catch (JsonException ex)
+            {
+                throw new KlarnaApiException($"Failed to deserialize Klarna API response: {ex.Message}", ex);
+            }
+        }
+    }
+}

src/Smartstore.Modules/Smartstore.Klarna/Client/KlarnaMerchantUrls.cs

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+using Newtonsoft.Json;
+
+namespace Smartstore.Klarna.Client
+{
+    public class KlarnaMerchantUrls
+    {
+        [JsonProperty("confirmation")]
+        public string Confirmation { get; set; }
+
+        [JsonProperty("notification")]
+        public string Notification { get; set; }
+
+        [JsonProperty("push")]
+        public string Push { get; set; }
+    }
+}

src/Smartstore.Modules/Smartstore.Klarna/Client/KlarnaOrderLine.cs

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+using Newtonsoft.Json;
+
+namespace Smartstore.Klarna.Client
+{
+    public class KlarnaOrderLine
+    {
+        [JsonProperty("type")]
+        public string Type { get; set; } // e.g. "physical", "digital", "shipping_fee"
+
+        [JsonProperty("name")]
+        public string Name { get; set; }
+
+        [JsonProperty("quantity")]
+        public int Quantity { get; set; }
+
+        [JsonProperty("unit_price")]
+        public long UnitPrice { get; set; } // In cents
+
+        [JsonProperty("total_amount")]
+        public long TotalAmount { get; set; } // In cents
+
+        [JsonProperty("tax_rate")]
+        public long TaxRate { get; set; } // In percent * 100, e.g., 2500 for 25%
+
+        [JsonProperty("total_tax_amount")]
+        public long TotalTaxAmount { get; set; } // In cents
+    }
+}

src/Smartstore.Modules/Smartstore.Klarna/Client/SessionDetailsResponse.cs

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+using Newtonsoft.Json;
+
+namespace Smartstore.Klarna.Client
+{
+    public class SessionDetailsResponse : KlarnaApiResponse
+    {
+        // Properties for session details
+        // e.g., Status, OrderAmount, OrderLines, ClientToken, ExpiresAt etc.
+        [JsonProperty("session_id")]
+        public string SessionId { get; set; }
+
+        [JsonProperty("client_token")]
+        public string ClientToken { get; set; }
+
+        [JsonProperty("status")]
+        public string Status { get; set; }
+
+        [JsonProperty("order_amount")]
+        public long OrderAmount { get; set; }
+
+        [JsonProperty("order_lines")]
+        public KlarnaOrderLine[] OrderLines { get; set; }
+    }
+}

src/Smartstore.Modules/Smartstore.Klarna/Client/UpdateSessionRequest.cs

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+using Newtonsoft.Json;
+
+namespace Smartstore.Klarna.Client
+{
+    public class UpdateSessionRequest : KlarnaApiRequest
+    {
+        // Properties for updating a session
+        // e.g., OrderAmount, OrderLines etc. (similar to CreateSessionRequest)
+        [JsonProperty("order_amount")]
+        public long OrderAmount { get; set; }
+
+        [JsonProperty("order_lines")]
+        public KlarnaOrderLine[] OrderLines { get; set; }
+    }
+}