Snooze GitHub issue classification when model provider is unavailable

When a model provider returns billing, quota, suspension, or rate-limit errors during GitHub issue classification or revision summary work, Hive no longer treats those jobs as failed. Provider availability failures are now snoozed for a backoff period so the affected issues remain eligible for classification once the provider recovers, and provider errors are sanitized so prompt or request body content does not leak into logs or background job errors. Unexpected provider failures are still surfaced, now only with sanitized metadata.