Handle Sentry envelope transport failures gracefully

[cursor]

Summary

• catch requests.RequestException in GithubClient._send_envelope so outbound Sentry ingestion failures do not crash webhook handling
• add a request timeout for envelope POSTs to avoid hanging request threads on unreachable DSN hosts
• include host-focused logging for failed envelope delivery attempts
• add regression tests verifying connection failures and HTTP 500 responses from Sentry return None instead of raising

Validation

• python3 -m pytest tests/test_github_sdk.py tests/test_web_app_handler.py
• Result: 17 passed, 1 skipped

Context

This addresses Sentry issue SENTRY-GITHUB-ACTIONS-APP-77 (https://sentry.sentry.io/issues/7354001476/).