RestApiTool raises uncaught KeyError when a required path param is omitted #7282

Description

FunctionTool returns {"error": ...} when mandatory args are missing so the model can retry.

RestApiTool maps filled args into path_params, then does self.endpoint.path.format(**path_params) in _prepare_request_params. An omitted required path parameter leaves a template key unset → KeyError (original OpenAPI name, e.g. 'userId').

RestApiTool.call only catches TimeoutException / HTTPStatusError, so the agent invocation aborts instead of returning a retryable tool error.

Location: google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py

Environment

• google-adk==2.9.2
• Offline unit repro (no live HTTP)

Repro

# RestApiTool for path /users/{userId} with required path param userId / py_name user_id
# Call with args {} (or missing user_id)
# → KeyError: 'userId'

Observed (google-adk==2.9.2)

• path /users/{userId}, args {} → KeyError: 'userId'
• py_names for the param include user_id (declaration expects snake_case), but missing value still crashes via format

Expected

Missing required path params should return a structured tool error (mirror FunctionTool._get_mandatory_args) so the model can retry, not abort the agent run.

Suggested fix

Before format, verify every {placeholder} in the path is present; on miss return {"error": "missing path parameter …"}. Optionally catch KeyError/ValueError from _prepare_request_params in call.

Notes

Nearby RestApiTool crashes #7164 (header type) and #7206 (non-UTF-8 body) are different sites. Happy to open a PR.