The Discord gateway heartbeat stalled ('Shard ID None heartbeat blocked
for more than N seconds') because _handoff_watcher polled the synchronous,
blocking SQLite-backed SessionDB directly on the asyncio event loop every
2s. Each list_pending/claim/complete/fail call performed blocking disk I/O
on the loop thread, starving the Discord heartbeat coroutine.
Wrap every blocking SessionDB call inside the watcher loop in
asyncio.to_thread(...) so the SQLite work runs on a worker thread and the
event loop (and heartbeat) stays responsive. These four call sites are the
only synchronous self._session_db.* calls inside the watcher loop body.
Adds tests/gateway/test_handoff_watcher_async_db.py asserting the watcher
offloads its SessionDB calls via asyncio.to_thread (mutation-survivable:
reverting any to_thread wrap fails the corresponding assertion).
Fixes#40695
Co-authored-by: kshitijk4poor <82637225+kshitijk4poor@users.noreply.github.com>
