fix: close late scheduler poll during drain

tests/scheduling/test_lifecycle.py

@@ -457,6 +457,23 @@ async def test_cancelled_resume_release_waits_for_lock_cleanup(
         await service.stop()
 
 
+async def test_poll_once_rejects_after_stop_begins(tmp_path: Path) -> None:
+    """A late manual tick cannot enter the scheduler during drain."""
+    intended = ts(2026, 9, 8, 12, 0)
+    service = _service(tmp_path, ScriptedRuntime("complete"))
+    try:
+        await service.start()
+        service.schedule_store.create_schedule(_sched_model("a", intended))
+        service._stopping = True
+
+        with pytest.raises(SchedulerStartupError, match="stopping"):
+            await service.poll_once(intended + timedelta(seconds=1))
+        assert service.run_store.list_runs() == []
+    finally:
+        service._stopping = False
+        await service.stop()
+
+
 async def test_scheduled_interrupt_stays_resumable(tmp_path: Path) -> None:
     intended = ts(2026, 9, 8, 12, 0)
     service = _service(tmp_path, ScriptedRuntime("interrupt"))