The synchronous llm_call() runs in FastAPI's threadpool (sync route
handlers such as POST /sessions/auto-sort), while llm_call_async() runs
on the event loop. Both mutate the module-level _response_cache,
_host_fails and _dead_hosts dicts, so these are touched from multiple OS
threads concurrently. Two races result:
- _set_cached_response() snapshots 64 keys then deletes them with
`del _response_cache[key]`; if another thread evicts the same key
first, the del raises KeyError mid-eviction. Switched to
pop(key, None).
- _mark_host_dead() does get()+1+set() on _host_fails with no lock, so
concurrent connect failures lose increments and a genuinely dead host
can stay under its cooldown threshold. Guarded the host-health maps
with a threading.Lock (also applied to _is_host_dead / _clear_host_dead
for consistent reads).
Adds tests/test_llm_core_concurrency.py with deterministic regression
tests (phantom snapshot key for the eviction race; a slow-read dict that
forces the lost-update window for the counter). Both fail on the
unpatched code and pass with the fix.
