The liveness handler called SchedulerHealth(), which refreshes provider
instance caches under the monitor's write lock and snapshots queues and
breakers on every probe — it only needed the dead-letter count. Add
Monitor.DeadLetterCount() (queue-local mutex only) and use it.
The hot-dev watchdog killed the backend after 2 missed 3s probes; on a
dev box running race-detector test suites and builds that is routine
CPU starvation, and each kill destroyed any in-flight Assistant chat
stream (observed repeatedly during the chat-feel audit). Default the
miss threshold to 4 (~20s) — genuine wedges still die quickly.
rcourtman
b1114f4986