Engine: a hung LLM stream never times out — the turn and its busy registration leak forever #1739

The episode (2026-10-09, live-diagnosed on shard 3): a session's turn started normally (streams at 09:46-09:47Z, step 3), then went silent with the engine otherwise healthy — /config 5ms, idle CPU, and crucially no provider socket left: the baseten connection died without an error ever surfacing to the stream await. The turn never unwound: the run-coordinator held the session busy for 8 hours while its work had long finished, the panel rendered eternal "thinking," and only an explicit POST /session/{id}/abort cleared it (cancel + the errored message at 17:32:25Z).

The gap: the transport work bounded the provider response (#1716's 64MB cap) and the question wait is now cancellable (#1730), but the LLM stream await itself has no idle/read timeout: a provider socket that dies without a clean FIN/EOF (tunnel drop, NAT timeout, LB idle-kill) leaves the read parked forever, and with it the turn, its coordinator registration, and the session's UI state.

Fix shape: an idle timeout on the stream read (configurable, generous — long generations legitimately pause between chunks; reasoning deltas arrive continuously, so a 60-120s idle ceiling is safe) that fails the step with a transport error — the existing error path then unwinds the turn, publishes the failure state, and frees the coordinator. This is the last member of the lifecycle family: tools (#1708), questions (#1730), and now the stream itself.

Reproduction sketch: start a turn, kill the provider connection mid-stream without a clean close (e.g. drop the TCP connection via a firewall rule or a proxy that blackholes it), observe the session stay busy indefinitely and the step never error.

Part of: the #775/#1717 lifecycle seams. Operational note: the frontdoor's liveness watchdogs catch wedged engines, not wedged turns — a session-level stuck-busy detector in the frontdoor (busy with zero log activity + zero event flow for N minutes → surface a warning card) would bridge the gap until this lands.