GitLab

- Empty stream responses now return UpstreamFailoverError instead of
  plain 502, triggering automatic account switching (up to 10 retries)
- Add tempUnscheduleEmptyResponse: accounts returning empty responses
  are temp-unscheduled for 30 minutes
- Apply to both Claude and Gemini non-streaming paths
- Align googleConfigErrorCooldown from 60m to 30m for consistency