Handle GitHub token cleanup network failures without masking root errors

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Summary

• wrap GitHub App token revocation in GithubAppToken.get_token() with best-effort error handling
• log revocation failures instead of raising, so the original workflow processing exception is preserved
• add regression tests for successful cleanup and for cleanup network failure while propagating the original exception

Validation

• python3 -m pytest tests/test_github_app.py

Sentry incident

• Addresses SENTRY-GITHUB-ACTIONS-APP-72 (ConnectionError: ('Connection aborted.', ConnectionResetError(104, 'Connection reset by peer')))
• Root cause: cleanup requests.delete(...) in context manager finally could raise a second network exception and mask the true upstream failure path