Sanitize API fallback failure comments

[Pigbibi]

Summary

• summarize provider HTTP errors without raw response bodies
• redact API-key-like values before writing fallback warnings to issue comments
• add regression coverage for failed reviewer warning sanitization

Tests

• python3 -m ruff check scripts/run_monthly_codex_audit.py tests/test_run_monthly_codex_audit.py
• python3 -m unittest tests.test_run_monthly_codex_audit.RunMonthlyCodexAuditTests.test_run_configured_api_reviews_sanitizes_failed_reviewer_errors tests.test_run_monthly_codex_audit.RunMonthlyCodexAuditTests.test_service_failure_classification_ignores_source_code_secret_words
• python3 -m unittest discover -s tests
• git diff --check

[github-actions]

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