Brokered inference (both the streaming and buffered front executors) hard-failed
in the window where no engine/model was ready yet — e.g. engine still warming up,
or an OOM-triggered evict+reload in progress. Over the broker that not-ready reply
lands at the client as a single empty SSE chunk (the "empty reply / looks like a
dead session" symptom), even though a direct API request to the same engine just
waits (the engine's own /v1/chat/completions handler retries ~5min for a load).

Make the broker paths behave like the direct path: when pick_engine returns no
ready engine (or the engine returns a non-200 not-ready status), wait and retry,
attempt-bound (6 × 5s), and only surface an error once retries are exhausted. A
non-200 means no tokens were generated, so re-sending the body is safe (no
duplicate output).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RdMufYvtTbtGDWsiZVoXce