style: add fix for TRY301 lint errors

[holtskinner]

https://docs.astral.sh/ruff/rules/raise-within-try/

[gemini-code-assist]

Summary of Changes

Hello @holtskinner, 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 PR addresses TRY301 linting errors by refactoring error handling and post-processing logic in several asynchronous request handlers. Specifically, raise ServerError calls and subsequent result processing are moved out of try blocks to ensure they are not inadvertently caught by broad except Exception clauses, leading to clearer and more explicit error management.

Highlights

• Error Handling Refactoring: Restructured on_message_send in default_request_handler.py to move ServerError raises and related post-processing logic outside the try...except block, ensuring specific error handling is not masked by a general exception catch.
• JSON-RPC Handler Updates: Applied similar refactoring to on_cancel_task and on_get_task within jsonrpc_handler.py, relocating success response preparation and TaskNotFoundError raises to occur after the main try...except block.
• Lint Rule Compliance: Fixed TRY301 lint errors by ensuring raise statements are not directly within try blocks that have a generic except Exception clause, promoting more precise exception handling.

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[gemini-code-assist]

Code Review

This pull request refactors the code to fix TRY301 lint errors. The changes involve moving raise ServerError(...) statements out of try blocks to avoid them being caught by broad except clauses.

The modifications in src/a2a/server/request_handlers/default_request_handler.py correctly move the result validation logic outside the try...finally block. This not only resolves the lint issue but also makes the Agent execution failed log message more precise, as it now only triggers for exceptions during the agent's event consumption.

Similarly, the changes in src/a2a/server/request_handlers/jsonrpc_handler.py for on_cancel_task and on_get_task correctly refactor the error handling for cases where a task is not found, preserving the original logic while adhering to linting rules.

Overall, the changes are well-implemented, targeted, and improve code quality. I have no further comments.