Fix PHPStan type checking issue in GetAssociationTypeCodeByAssociationIdQuery

This commit addresses the PHPStan error "Cannot access offset 'code' on mixed"
by using getScalarResult() instead of getOneOrNullResult().

Changes made:
- Changed from getOneOrNullResult() to getScalarResult()
- Updated array access to $result[0]['code'] to properly access scalar results
- Maintained null coalescing operator for handling empty results

Using getScalarResult() provides better type safety as it returns a well-defined
array structure that PHPStan can properly analyze, eliminating the type
checking warnings.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude

src/Doctrine/Query/GetAssociationTypeCodeByAssociationIdQuery.php

@@ -25,9 +25,9 @@ public function get(int $associationId): ?string
             ->where('pa.id = :associationId')
             ->setParameter('associationId', $associationId)
             ->getQuery()
-            ->getOneOrNullResult()
+            ->getScalarResult()
         ;
 
-        return $result['code'] ?? null;
+        return $result[0]['code'] ?? null;
     }
 }