Fix: Add CustomizeDataReaderExpression to convert string to MemoryStream

MySQL stores JSON as strings, but EF Core expects MemoryStream for JSON complex types.
Added CustomizeDataReaderExpression override to convert string to MemoryStream.
Also kept GetDataReaderMethod override to use GetString.

This fixes the "No coercion operator is defined between types 'System.String' and 'System.IO.MemoryStream'" error.

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

Author: Copilot

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

@@ -4,8 +4,11 @@
 using System;
 using System.Collections.Generic;
 using System.Data.Common;
+using System.IO;
 using System.Linq;
+using System.Linq.Expressions;
 using System.Reflection;
+using System.Text;
 using JetBrains.Annotations;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.ChangeTracking;
@@ -130,6 +133,37 @@ protected override void ConfigureParameter(DbParameter parameter)
         public override MethodInfo GetDataReaderMethod()
             => _getString;
 
+        /// <summary>
+        /// Customizes the data reader expression for JSON types.
+        /// MySQL stores JSON as strings, but EF Core expects MemoryStream for complex JSON types.
+        /// This method creates an expression that converts the string to a MemoryStream containing UTF-8 encoded bytes.
+        /// </summary>
+        public override Expression CustomizeDataReaderExpression(Expression expression)
+        {
+            // For JSON complex types, EF Core expects a MemoryStream containing JSON data.
+            // MySQL returns JSON as strings, so we need to convert: string -> byte[] -> MemoryStream
+            
+            if (expression.Type == typeof(string))
+            {
+                // Convert string to MemoryStream: new MemoryStream(Encoding.UTF8.GetBytes(stringValue))
+                var getBytes = typeof(System.Text.Encoding).GetProperty(nameof(System.Text.Encoding.UTF8))!
+                    .GetMethod!;
+                var getBytesMethod = typeof(System.Text.Encoding).GetMethod(
+                    nameof(System.Text.Encoding.GetBytes),
+                    new[] { typeof(string) })!;
+                var memoryStreamCtor = typeof(System.IO.MemoryStream).GetConstructor(new[] { typeof(byte[]) })!;
+
+                return Expression.New(
+                    memoryStreamCtor,
+                    Expression.Call(
+                        Expression.Property(null, getBytes),
+                        getBytesMethod,
+                        expression));
+            }
+
+            return base.CustomizeDataReaderExpression(expression);
+        }
+
         void IMySqlCSharpRuntimeAnnotationTypeMappingCodeGenerator.Create(
             CSharpRuntimeAnnotationCodeGeneratorParameters codeGeneratorParameters,
             CSharpRuntimeAnnotationCodeGeneratorDependencies codeGeneratorDependencies)