fix(gateway): exclude _run_restart from _background_tasks to prevent zombie on /restart

When request_restart() adds _run_restart to _background_tasks, _stop_impl
later cancels all entries in that set.  Since _run_restart is awaiting
_stop_task at that point, the CancelledError propagates into _stop_impl,
interrupting cleanup before _shutdown_event.set() and _exit_code = 75
execute.  This leaves the gateway as a zombie (alive but disconnected) or
exiting with code 0 instead of 75, preventing systemd Restart=on-failure
from restarting the service.

Fix: don't add _run_restart to _background_tasks — it self-terminates in
~50ms and needs no lifecycle management.

Fixes #12875

gateway/run.py

@@ -5520,9 +5520,13 @@ class GatewayRunner(GatewayAuthorizationMixin, GatewayKanbanWatchersMixin, Gatew
             await asyncio.sleep(0.05)
             await self.stop(restart=True, detached_restart=detached, service_restart=via_service)
 
-        task = asyncio.create_task(_run_restart())
-        self._background_tasks.add(task)
-        task.add_done_callback(self._background_tasks.discard)
+        # _run_restart is a short-lived self-terminating task (calls stop()
+        # then returns).  Don't add it to _background_tasks — _stop_impl
+        # cancels all entries in that set, which would cancel _run_restart
+        # while it's awaiting _stop_task, propagating CancelledError into
+        # _stop_impl and preventing _shutdown_event.set() / _exit_code = 75.
+        # See #12875.
+        asyncio.create_task(_run_restart())
         return True
 
     # Drain-timeout reasons set by _stop_impl() when a still-running turn is

tests/gateway/test_restart_drain.py

@@ -181,11 +181,20 @@ async def test_request_restart_is_idempotent():
     runner, _adapter = make_restart_runner()
     runner.stop = AsyncMock()
 
-    assert runner.request_restart(detached=True, via_service=False) is True
-    first_task = next(iter(runner._background_tasks))
-    assert runner.request_restart(detached=True, via_service=False) is False
+    # Patch create_task to capture the restart task (it's no longer in
+    # _background_tasks — see #12875).
+    _captured = []
+    _orig_create_task = asyncio.create_task
+    def _capture(coro, **kw):
+        t = _orig_create_task(coro, **kw)
+        _captured.append(t)
+        return t
+    with pytest.MonkeyPatch.context() as mp:
+        mp.setattr(asyncio, "create_task", _capture)
+        assert runner.request_restart(detached=True, via_service=False) is True
+        assert runner.request_restart(detached=True, via_service=False) is False
 
-    await first_task
+    await _captured[0]
 
     runner.stop.assert_awaited_once_with(
         restart=True, detached_restart=True, service_restart=False