Fix issue when writing custom values to STJ

This new mechanism is more flexible and future-proof. In addition, it actually works with Ulid, unlike the previous one.

src/StructId.FunctionalTests/Functional.cs

@@ -24,6 +24,23 @@ public record User(UserId Id, string Name, Wallet Wallet);
 
 public class FunctionalTests(ITestOutputHelper output)
 {
+    [Fact]
+    public void JsonConversion()
+    {
+        var product = new Product(ProductId.New(), "Product");
+
+        var json = System.Text.Json.JsonSerializer.Serialize(product);
+        var product2 = System.Text.Json.JsonSerializer.Deserialize<Product>(json);
+
+        Assert.Equal(product, product2);
+
+        var user = new User(UserId.New(1), "User", new Wallet(WalletId.New("1234"), "Wallet"));
+        json = System.Text.Json.JsonSerializer.Serialize(user);
+        var user2 = System.Text.Json.JsonSerializer.Deserialize<User>(json);
+
+        Assert.Equal(user, user2);
+    }
+
     [Fact]
     public void EqualityTest()
     {

src/StructId.FunctionalTests/UlidTests.cs

@@ -1,5 +1,6 @@
 using System.Collections.Concurrent;
 using System.Data;
+using System.Text.Json;
 using Dapper;
 using Microsoft.Data.Sqlite;
 using Microsoft.EntityFrameworkCore;
@@ -68,6 +69,17 @@ public UlidToStringConverter(ConverterMappingHints? mappingHints = null)
 
 public class UlidTests
 {
+    [Fact]
+    public void JsonConversion()
+    {
+        var product = new UlidProduct(UlidId.New(), "Product");
+
+        var json = JsonSerializer.Serialize(product);
+        var product2 = JsonSerializer.Deserialize<UlidProduct>(json);
+
+        Assert.Equal(product, product2);
+    }
+
     [Fact]
     public void Dapper()
     {

src/StructId/StructIdConverters.JsonConverter.cs

@@ -11,10 +11,15 @@ public static partial class StructIdConverters
 {
     public class SystemTextJsonConverter<TSelf, TValue> : JsonConverter<TSelf>
         where TSelf : IStructId<TValue>, INewable<TSelf, TValue>
-        where TValue: struct, IParsable<TValue>
+        where TValue : struct, IParsable<TValue>
     {
         public override TSelf Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
-            => TSelf.New(TValue.Parse(reader.GetString() ?? throw new FormatException("Unsupported null value for struct id."), CultureInfo.InvariantCulture));
+        {
+            if (reader.TokenType == JsonTokenType.String)
+                return TSelf.New(TValue.Parse(reader.GetString() ?? throw new FormatException("Unsupported null value for struct id."), CultureInfo.InvariantCulture));
+
+            return TSelf.New(JsonSerializer.Deserialize<TValue>(ref reader, options));
+        }
 
         public override void Write(Utf8JsonWriter writer, TSelf value, JsonSerializerOptions options)
         {
@@ -24,7 +29,7 @@ public override void Write(Utf8JsonWriter writer, TSelf value, JsonSerializerOpt
                     writer.WriteStringValue(guid);
                     break;
                 case TValue inner:
-                    writer.WriteRawValue(inner.ToString());
+                    JsonSerializer.Serialize(writer, inner, options);
                     break;
                 default:
                     throw new InvalidOperationException("Unsupported value type.");