- Treat status 499 as terminal non-retryable error in both priority and
round-robin combo loops — no fallback to other models when client is gone
- Propagate AbortSignal from request into handleComboChat so the combo
loop can detect client disconnects before starting new model attempts
- Make retry/fallback delays abort-aware via signal.addEventListener
- Add 5 unit tests covering 499 early-exit, signal.aborted pre-check,
multi-model abort, 502 contrast behavior, and abort-during-wait
