fix: type guard cached schema description

Author: Sakeeb91

app/agent/engine.py

@@ -342,7 +342,7 @@ async def _get_schema_description(self, db_context: DatabaseContext) -> str:
             cached_payload = await get_cached_schema(db_context.database_id)
             if isinstance(cached_payload, dict):
                 cached_description = cached_payload.get("serialized_schema")
-                if cached_description:
+                if isinstance(cached_description, str) and cached_description:
                     self._schema_cache[db_context.database_id] = cached_description
                     return cached_description