fix: harden Polygon operation validation

site/routes/instances_routes.py

@@ -80,6 +80,9 @@ def create_instance():
     service_id = body.get("serviceId")
     display_name = body.get("displayName", "unnamed")
 
+    if not isinstance(display_name, str):
+        return jsonify({"error": "displayName must be a string"}), 400
+
     # Валидация: serviceId обязателен
     if service_id is None:
         return jsonify({"error": "serviceId is required"}), 400