Improve perf of base64 encoding check on a consumer hot path

src/Confluent.SchemaRegistry/CachedSchemaRegistryClient.cs

@@ -592,34 +592,17 @@ private bool checkSchemaMatchesFormat(string format, string schemaString)
             // if a format isn't specified, then assume text is desired.
             if (format == null)
             {
-                try
-                {
-                    Convert.FromBase64String(schemaString);
-                }
-                catch (Exception)
-                {
-                    return true; // Base64 conversion failed, infer the schemaString format is text.
-                }
-
-                return false; // Base64 conversion succeeded, so infer the schamaString format is base64.
+                // If schemaString is not Base64, infer the schemaString format is text.
+                return !Utils.IsBase64String(schemaString);
             }
             else
             {
                 if (format != "serialized")
                 {
                     throw new ArgumentException($"Invalid schema format was specified: {format}.");
                 }
-
-                try
-                {
-                    Convert.FromBase64String(schemaString);
-                }
-                catch (Exception)
-                {
-                    return false;
-                }
-
-                return true;
+
+                return Utils.IsBase64String(schemaString);
             }
         }
 

src/Confluent.SchemaRegistry/Utils.cs

@@ -20,6 +20,9 @@
 using System.Linq;
 using Confluent.Kafka;
 
+#if NET8_0_OR_GREATER
+using System.Buffers.Text;
+#endif
 
 namespace Confluent.SchemaRegistry
 {
@@ -70,5 +73,22 @@ public static bool ListEquals<T>(IList<T> a, IList<T> b)
             if (a == null || b == null) return false;
             return a.SequenceEqual(b);
         }
+
+        internal static bool IsBase64String(string value)
+        {
+#if NET8_0_OR_GREATER
+            return Base64.IsValid(value);
+#else
+            try
+            {
+                _ = Convert.FromBase64String(value);
+                return true;
+            }
+            catch (FormatException)
+            {
+                return false;
+            }
+#endif
+        }
     }
 }