Fix: Match PostgreSQL implementation - use dbType: null

Changed MySqlStructuralJsonTypeMapping to match PostgreSQL's Npgsql implementation:
1. Use `dbType: null` instead of `DbType.String` in base constructor
2. Use inline Expression.New() for CustomizeDataReaderExpression like Npgsql
3. Match PostgreSQL's static field pattern for reflection lookups

PostgreSQL successfully uses JsonTypePlaceholder as ClrType with these exact patterns.
The dbType: null parameter is critical for proper SQL column projection.

Co-authored-by: renemadsen <76994+renemadsen@users.noreply.github.com>

Author: Copilot

src/EFCore.MySql/Storage/Internal/MySqlStructuralJsonTypeMapping.cs

@@ -15,19 +15,26 @@ namespace Pomelo.EntityFrameworkCore.MySql.Storage.Internal
     /// <summary>
     /// Type mapping for complex JSON types (types mapped with .ToJson()).
     /// This mapping handles the conversion from MySQL's string-based JSON to MemoryStream expected by EF Core.
+    /// Similar to Npgsql's NpgsqlStructuralJsonTypeMapping.
     /// </summary>
     public class MySqlStructuralJsonTypeMapping : JsonTypeMapping
     {
-        private static readonly MethodInfo _createUtf8StreamMethod
-            = typeof(MySqlStructuralJsonTypeMapping).GetMethod(nameof(CreateUtf8Stream), new[] { typeof(string) });
-
         private static readonly MethodInfo _getStringMethod
             = typeof(DbDataReader).GetRuntimeMethod(nameof(DbDataReader.GetString), new[] { typeof(int) });
 
+        private static readonly PropertyInfo _utf8Property
+            = typeof(Encoding).GetProperty(nameof(Encoding.UTF8));
+
+        private static readonly MethodInfo _getBytesMethod
+            = typeof(Encoding).GetMethod(nameof(Encoding.GetBytes), new[] { typeof(string) });
+
+        private static readonly ConstructorInfo _memoryStreamConstructor
+            = typeof(MemoryStream).GetConstructor(new[] { typeof(byte[]) });
+
         public static MySqlStructuralJsonTypeMapping Default { get; } = new("json");
 
         public MySqlStructuralJsonTypeMapping(string storeType)
-            : base(storeType, typeof(JsonTypePlaceholder), System.Data.DbType.String)
+            : base(storeType, typeof(JsonTypePlaceholder), dbType: null)
         {
             Console.WriteLine($"[DEBUG] MySqlStructuralJsonTypeMapping created - StoreType: {storeType}, ClrType: JsonTypePlaceholder");
         }
@@ -47,19 +54,19 @@ public override MethodInfo GetDataReaderMethod()
             return _getStringMethod;
         }
 
-        /// <summary>
-        /// Converts the string read from MySQL to a MemoryStream for EF Core's JSON processing.
-        /// </summary>
-        public static MemoryStream CreateUtf8Stream(string json)
-            => new MemoryStream(Encoding.UTF8.GetBytes(json ?? string.Empty));
-
         /// <summary>
         /// Customizes the data reader expression to convert string to MemoryStream.
+        /// Creates: new MemoryStream(Encoding.UTF8.GetBytes(stringValue))
         /// </summary>
         public override Expression CustomizeDataReaderExpression(Expression expression)
         {
             Console.WriteLine("[DEBUG] MySqlStructuralJsonTypeMapping.CustomizeDataReaderExpression() called - converting string to MemoryStream");
-            return Expression.Call(_createUtf8StreamMethod, expression);
+            return Expression.New(
+                _memoryStreamConstructor,
+                Expression.Call(
+                    Expression.Property(null, _utf8Property),
+                    _getBytesMethod,
+                    expression));
         }
 
         protected override RelationalTypeMapping Clone(RelationalTypeMappingParameters parameters)