fix(gateway): keep the drain-marker read off the event loop

_drain_control_watcher polls drain_requested() once a second, and that
does a synchronous Path.read_text() on the marker file. At a 1s cadence
that is ~86k blocking disk reads a day sitting on the event loop.

It stays invisible until the host is under I/O pressure, at which point
a single read can stall for 30s+ and take every platform heartbeat down
with it. Observed as repeated "discord.gateway: Shard ID None heartbeat
blocked for more than 30 seconds" followed by the Discord websocket
dropping, with the gateway still reporting active.

Move the call to asyncio.to_thread so the read blocks a worker thread
instead of the loop. Semantics are unchanged: drain_control never
raises, the surrounding try/except still covers it, and the sequential
await means at most one probe is in flight at a time.

gateway/run.py

@@ -9876,7 +9876,12 @@ class GatewayRunner(GatewayAuthorizationMixin, GatewayKanbanWatchersMixin, Gatew
 
         while self._running:
             try:
-                if drain_requested():
+                # drain_requested() does a synchronous read_text() on the
+                # marker file. At this 1s cadence that puts a blocking disk
+                # read on the event loop ~86k times a day; when the host is
+                # under I/O pressure a single read can stall for 30s+ and
+                # take every platform heartbeat down with it. Off-thread it.
+                if await asyncio.to_thread(drain_requested):
                     self._enter_external_drain()
                     # API and cron work live outside messaging's
                     # _running_agents map. Refresh the aggregate while an