Fix: Check JsonTypePlaceholder BEFORE store type lookups

Moved the JsonTypePlaceholder check to the very beginning of FindRawMapping,
before any store type lookups. This matches SQL Server's implementation and
ensures the structural JSON type mapping is returned correctly.

The previous placement after store type lookups was causing EF Core to find
regular JSON mappings first, leading to SQL generation issues with empty
columns in SELECT statements.

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

Author: Copilot

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

@@ -312,6 +312,15 @@ private RelationalTypeMapping FindRawMapping(RelationalTypeMappingInfo mappingIn
             var storeTypeName = mappingInfo.StoreTypeName;
             var storeTypeNameBase = mappingInfo.StoreTypeNameBase;
 
+            // Special case for JSON columns: EF Core passes JsonTypePlaceholder as the CLR type
+            // when creating JSON columns for complex types/collections. Return our structural JSON mapping.
+            // This MUST be checked first, before any store type lookups, similar to SQL Server's implementation.
+            if (clrType?.Name == "JsonTypePlaceholder")
+            {
+                Console.WriteLine($"[DEBUG] MySqlTypeMappingSource: Detected JsonTypePlaceholder - returning MySqlStructuralJsonTypeMapping");
+                return _jsonStructural;
+            }
+
             if (storeTypeName != null)
             {
                 // First look for the fully qualified store type name.
@@ -323,14 +332,6 @@ private RelationalTypeMapping FindRawMapping(RelationalTypeMappingInfo mappingIn
                     // If a CLR type was provided, look for a mapping between the store and CLR types. If none is found,
                     // fail immediately.
 
-                    // Special case for JSON columns: EF Core passes JsonTypePlaceholder as the CLR type
-                    // 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;
-                    }
-                    
                     return clrType == null
                         ? mappings[0]
                         : mappings.FirstOrDefault(m => m.ClrType == clrType);