Debug: Add Console.WriteLine logging to track type mapping usage

Added debug logging to:
- MySqlStructuralJsonTypeMapping: Constructor, Clone, GetDataReaderMethod, CustomizeDataReaderExpression
- MySqlJsonTypeMapping: GetDataReaderMethod, CustomizeDataReaderExpression
- MySqlTypeMappingSource: When JsonTypePlaceholder is detected and structural mapping is returned

This will help diagnose whether the structural type mapping is being used correctly
or if the SQL generation is still seeing different type mappings that break
column projection.

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

Author: Copilot

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

@@ -130,7 +130,10 @@ protected override void ConfigureParameter(DbParameter parameter)
         /// MySQL stores JSON as strings, so we use GetString instead of the default GetFieldValue<T>.
         /// </summary>
         public override MethodInfo GetDataReaderMethod()
-            => _getString;
+        {
+            Console.WriteLine($"[DEBUG] MySqlJsonTypeMapping.GetDataReaderMethod() called - ClrType: {ClrType.Name} - returning DbDataReader.GetString");
+            return _getString;
+        }
 
         /// <summary>
         /// Customizes the data reader expression for JSON types.
@@ -139,6 +142,7 @@ public override MethodInfo GetDataReaderMethod()
         /// </summary>
         public override Expression CustomizeDataReaderExpression(Expression expression)
         {
+            Console.WriteLine($"[DEBUG] MySqlJsonTypeMapping.CustomizeDataReaderExpression() called - ClrType: {ClrType.Name} - no conversion");
             // For regular JSON columns, no conversion needed - just return the string
             return base.CustomizeDataReaderExpression(expression);
         }

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

@@ -29,18 +29,23 @@ private static readonly MethodInfo _getStringMethod
         public MySqlStructuralJsonTypeMapping(string storeType)
             : base(storeType, typeof(JsonTypePlaceholder), System.Data.DbType.String)
         {
+            Console.WriteLine($"[DEBUG] MySqlStructuralJsonTypeMapping created - StoreType: {storeType}, ClrType: JsonTypePlaceholder");
         }
 
         protected MySqlStructuralJsonTypeMapping(RelationalTypeMappingParameters parameters)
             : base(parameters)
         {
+            Console.WriteLine($"[DEBUG] MySqlStructuralJsonTypeMapping cloned - StoreType: {parameters.StoreType}");
         }
 
         /// <summary>
         /// MySQL stores JSON as strings, so we read using GetString.
         /// </summary>
         public override MethodInfo GetDataReaderMethod()
-            => _getStringMethod;
+        {
+            Console.WriteLine("[DEBUG] MySqlStructuralJsonTypeMapping.GetDataReaderMethod() called - returning DbDataReader.GetString");
+            return _getStringMethod;
+        }
 
         /// <summary>
         /// Converts the string read from MySQL to a MemoryStream for EF Core's JSON processing.
@@ -52,7 +57,10 @@ public static MemoryStream CreateUtf8Stream(string json)
         /// Customizes the data reader expression to convert string to MemoryStream.
         /// </summary>
         public override Expression CustomizeDataReaderExpression(Expression expression)
-            => Expression.Call(_createUtf8StreamMethod, expression);
+        {
+            Console.WriteLine("[DEBUG] MySqlStructuralJsonTypeMapping.CustomizeDataReaderExpression() called - converting string to MemoryStream");
+            return Expression.Call(_createUtf8StreamMethod, expression);
+        }
 
         protected override RelationalTypeMapping Clone(RelationalTypeMappingParameters parameters)
             => new MySqlStructuralJsonTypeMapping(parameters);

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

@@ -327,6 +327,7 @@ private RelationalTypeMapping FindRawMapping(RelationalTypeMappingInfo mappingIn
                     // when creating JSON columns for complex types/collections. Return our structural JSON mapping.
                     if (clrType?.Name == "JsonTypePlaceholder" && storeTypeName.Equals("json", StringComparison.OrdinalIgnoreCase))
                     {
+                        Console.WriteLine($"[DEBUG] MySqlTypeMappingSource: Detected JsonTypePlaceholder for JSON column - returning MySqlStructuralJsonTypeMapping");
                         return _jsonStructural;
                     }