`gateway.sticker_cache._save_cache` ends in `atomic_json_write` -> `os.replace`,
whose duration is unbounded under filesystem pressure. Its only production
caller is Telegram's `_handle_sticker` -- an inbound-message coroutine -- so
every sticker that missed the cache paid that rename inline on the event loop,
stalling every other adapter and every in-flight turn in the process for its
duration.
Adds `cache_sticker_description_async`, which dispatches the existing sync form
via `asyncio.to_thread`; the Telegram call site awaits it. The sync form keeps
its exact contract and is what the wrapper dispatches to, so it is unchanged for
non-loop callers.
`cache_sticker_description` is a read-modify-write (`_load_cache` -> merge ->
`_save_cache`). `atomic_json_write` makes each WRITE atomic, not the TRIPLE.
While the call was inline the event loop serialized every caller and the race
could not be observed; moving the write to a worker thread introduces real
concurrency, so `_CACHE_LOCK` is added in this same commit rather than deferred.
Tests use no wall-clock thresholds. The liveness witness is ORDERING: the rename
is held open on a barrier released only by a background timer, and the sibling
task must have ticked BEFORE that release. Verified RED first:
- revert `asyncio.to_thread` to an inline call -> 2 failed
("the cache write ran on the event-loop thread")
- revert `_CACHE_LOCK` -> the concurrency test fails on the DURABLE FILE:
"lost an entry ... keys = ['uid_a']"
`test_gate_proof_the_sync_form_does_block_the_loop` drives the identical barrier
through the sync form and asserts the loop DOES starve, so the liveness
assertion cannot pass vacuously.
GREEN: sticker off-loop + existing sticker cache tests -> 10 passed;
`tests/gateway -k "sticker or telegram"` -> 937 passed, 4 skipped; Ruff clean.
(cherry picked from commit 9752a9b1aba7536181d081cad9c1c32917a89680)
cb567242d7