motion_queuing: Verify nothing attempts to pause in flush callbacks

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/motion_queuing.py

@@ -141,6 +141,7 @@ class PrinterMotionQueuing:
         step_gen_time = max(want_step_gen_time, self.last_step_gen_time,
                             flush_time)
         # Invoke flush callbacks (if any)
+        with self.reactor.assert_no_pause():
             for cb in self.flush_callbacks:
                 cb(flush_time, step_gen_time)
         # Determine maximum history to keep