Critical fix: Revert to dbType: null and add ClrType property override

KEY FINDING from Npgsql comparison: Must use `dbType: null` (not DbType.String)!

Changes:
1. Reverted `dbType: System.Data.DbType.String` → `dbType: null` to match Npgsql exactly
2. Added explicit `ClrType` property override returning `typeof(JsonTypePlaceholder)`

Npgsql's NpgsqlStructuralJsonTypeMapping uses dbType: null, which is critical
for EF Core's SQL generator to properly handle JsonTypePlaceholder during column
projection. Using DbType.String was causing the empty column issue.

The explicit ClrType property override ensures the type is consistently returned
as JsonTypePlaceholder, preventing EF Core from inferring different types.

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

Author: Copilot

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

@@ -34,9 +34,9 @@ private static readonly ConstructorInfo _memoryStreamConstructor
         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($"[DEBUG] MySqlStructuralJsonTypeMapping created - StoreType: {storeType}, ClrType: JsonTypePlaceholder, DbType: String");
+            Console.WriteLine($"[DEBUG] MySqlStructuralJsonTypeMapping created - StoreType: {storeType}, ClrType: JsonTypePlaceholder, DbType: null");
         }
 
         protected MySqlStructuralJsonTypeMapping(RelationalTypeMappingParameters parameters)
@@ -72,6 +72,11 @@ public override Expression CustomizeDataReaderExpression(Expression expression)
         protected override RelationalTypeMapping Clone(RelationalTypeMappingParameters parameters)
             => new MySqlStructuralJsonTypeMapping(parameters);
 
+        /// <summary>
+        /// Override to ensure we return JsonTypePlaceholder type consistently
+        /// </summary>
+        public override Type ClrType => typeof(JsonTypePlaceholder);
+
         protected string EscapeSqlLiteral(string literal)
             => literal.Replace("'", "''");