Fixing Mypy error

Author: llalitkumarrr

src/google/adk/cli/fast_api.py

@@ -79,7 +79,7 @@ def __getitem__(self, app_name: str) -> str:
       try:
         loaded = self.agent_loader.load_agent(folder_name)
         if isinstance(loaded, App) and loaded.name == app_name:
-          return folder_name
+          return str(folder_name)
       except Exception:
         pass
     return app_name