Prevent cleanup failures from masking GitHub webhook errors

[cursor]

Summary

• make GitHub App token revocation in GithubAppToken.get_token() best-effort
• catch and log requests.RequestException during token cleanup so the original application error is preserved
• add focused regression tests for cleanup-failure behavior

Root cause addressed

SENTRY-GITHUB-ACTIONS-APP-75 showed that a network failure while revoking installation tokens (requests.delete(...) in the context manager finally block) could raise ConnectionError and mask the original business error (for example, the upstream 403 while fetching sentry_config.ini).

Validation

• Ran: python3 -m pytest -q tests/test_github_app.py tests/test_web_app_handler.py tests/test_sentry_config_file.py
• Result: pass (with one pre-existing skipped test in test_web_app_handler.py)