Handle GitHub token cleanup connection errors without masking root cause

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Summary

• harden GithubAppToken.get_token() cleanup so revocation network failures are treated as best-effort
• prevent requests.delete() failures in the context manager finally block from masking the original webhook processing exception
• add focused unit tests for normal revocation, cleanup connection errors, and exception-preservation behavior

Validation

• python3 -m pytest -q

Sentry

• Addresses error signature: ConnectionError: ('Connection aborted.', ConnectionResetError(104, 'Connection reset by peer')) in SENTRY-GITHUB-ACTIONS-APP-72