fix: avoid 500s when a2a cleanup is cancelled

[lithammer]

We hit a case where a client retried on 5xx responses, and the agent's Slack tool had already executed before the server returned 500. The request itself was effectively successful, but cleanup raised CancelledError, which bubbled into a 500 and triggered the retry. That led to duplicate Slack messages.

This change treats CancelledError during _cleanup_producer as non‑fatal so a completed request doesn't get turned into a 500 during cleanup. The result is that clients don't retry after work has already been done, avoiding duplicate side effects.

[Copilot]

Pull request overview

This PR prevents duplicate side effects when clients retry on 5xx responses by treating CancelledError during A2A cleanup as non-fatal. Previously, if cleanup was cancelled after a request completed successfully, the server would return a 500 error, triggering client retries and causing duplicate operations (e.g., multiple Slack messages).

Changes:

• Introduced SafeRequestHandler that catches CancelledError during cleanup and performs best-effort resource cleanup instead of propagating the error
• Replaced DefaultRequestHandler with SafeRequestHandler in the A2A application
• Added unit tests verifying that cleanup handles both cancelled and successful task scenarios

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
python/packages/kagent-adk/src/kagent/adk/_safe_request_handler.py	New handler class that catches CancelledError during cleanup and performs manual resource cleanup
python/packages/kagent-adk/src/kagent/adk/_a2a.py	Updated to use SafeRequestHandler instead of DefaultRequestHandler
python/packages/kagent-adk/tests/unittests/test_safe_request_handler.py	Added tests for both cancelled and normal cleanup scenarios

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