Fix: Add GetDataReaderMethod override to MySqlComplexJsonTypeMapping

The MySqlComplexJsonTypeMapping was missing the GetDataReaderMethod() override,
causing it to use the default behavior which led to SQL syntax errors.
Added the override to return GetString for reading JSON as strings from MySQL.

Also made _getString protected so it can be accessed by derived classes.

Fixes SQL syntax error in ComplexJsonProjectionMySqlTest.

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

Author: Copilot

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

@@ -54,6 +54,13 @@ protected override RelationalTypeMapping Clone(bool? noBackslashEscapes = null,
                 noBackslashEscapes ?? NoBackslashEscapes,
                 replaceLineBreaksWithCharFunction ?? ReplaceLineBreaksWithCharFunction);
 
+        /// <summary>
+        /// Returns the method to be used for reading JSON values from the database.
+        /// MySQL stores JSON as strings, so we use GetString.
+        /// </summary>
+        public override MethodInfo GetDataReaderMethod()
+            => _getString;
+
         /// <summary>
         /// For complex JSON, we ALWAYS convert string to MemoryStream.
         /// </summary>
@@ -127,7 +134,7 @@ protected override RelationalTypeMapping Clone(bool? noBackslashEscapes = null,
 
     public abstract class MySqlJsonTypeMapping : MySqlStringTypeMapping, IMySqlCSharpRuntimeAnnotationTypeMappingCodeGenerator
     {
-        private static readonly MethodInfo _getString
+        protected static readonly MethodInfo _getString
             = typeof(DbDataReader).GetRuntimeMethod(nameof(DbDataReader.GetString), new[] { typeof(int) });
 
         // Cache reflection lookups for performance