Fix: Use DbType.String instead of null to enable SQL column projection

Changed MySqlStructuralJsonTypeMapping constructor to pass `System.Data.DbType.String`
instead of `dbType: null` to match SQL Server's SqlServerStructuralJsonTypeMapping.

ROOT CAUSE: The empty column projections (`, ,`) were caused by passing `dbType: null`.
EF Core's SQL generator needs the DbType to determine how to project columns in SELECT
statements. Without it, the generator cannot map JsonTypePlaceholder to actual column
names, resulting in empty SQL fragments.

SQL Server uses `DbType.String` with storeType="nvarchar(max)" or "json".
PostgreSQL likely does the same (needs verification).

This should resolve the SQL syntax errors while maintaining the string→MemoryStream
conversion for data reading.

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), dbType: null)
+            : base(storeType, typeof(JsonTypePlaceholder), System.Data.DbType.String)
         {
-            Console.WriteLine($"[DEBUG] MySqlStructuralJsonTypeMapping created - StoreType: {storeType}, ClrType: JsonTypePlaceholder");
+            Console.WriteLine($"[DEBUG] MySqlStructuralJsonTypeMapping created - StoreType: {storeType}, ClrType: JsonTypePlaceholder, DbType: String");
         }
 
         protected MySqlStructuralJsonTypeMapping(RelationalTypeMappingParameters parameters)