fix: return 422 for invalid historyLength in REST get_task

[azilbersteinSFDC]

Replace unhandled int() conversion with Pydantic validation so non-numeric historyLength query params (e.g. ?historyLength=abc) return 422 InvalidParamsError instead of 500.

Description

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[gemini-code-assist]

Summary of Changes

Hello @azilbersteinSFDC, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the robustness of the get_task REST API by implementing proper validation for the historyLength query parameter. By leveraging Pydantic, the system now gracefully handles malformed input, providing a clear 422 error response to the client rather than an unexpected server error, thereby improving API reliability and user experience.

Highlights

• Improved Error Handling: The get_task REST endpoint now uses Pydantic validation for the historyLength query parameter, preventing unhandled int() conversion errors.
• Standardized Error Response: Invalid non-numeric historyLength query parameters will now correctly return a 422 Unprocessable Entity (InvalidParamsError) instead of a 500 Internal Server Error.
• New Test Coverage: A new test case was added to ensure the get_task endpoint properly returns a 422 status code for invalid historyLength inputs.

Changelog

• src/a2a/server/request_handlers/rest_handler.py
  • Imported ValidationError from pydantic for robust data validation.
  • Imported InvalidParamsError to provide specific error responses for invalid parameters.
  • Modified the on_get_task function to wrap TaskQueryParams instantiation in a try-except ValidationError block.
  • Replaced direct int() conversion of history_length_str with Pydantic's validation mechanism.
  • Introduced raising a ServerError with InvalidParamsError when historyLength fails Pydantic validation.
• tests/server/apps/rest/test_rest_fastapi_app.py
  • Added a new asynchronous test test_get_task_invalid_history_length_returns_422.
  • The new test verifies that a GET request to /v1/tasks/some-task-id with a non-numeric historyLength query parameter returns an HTTP 422 status code.
  • The test also asserts that the response body contains a message indicating an issue with 'historylength'.

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[gemini-code-assist]

Code Review

This pull request effectively addresses the issue of unhandled int() conversion for the historyLength query parameter in the get_task REST endpoint. By integrating Pydantic validation, the system now correctly returns a 422 InvalidParamsError for non-numeric inputs, significantly improving error handling and API robustness. The addition of a dedicated test case ensures this new behavior is properly validated.