[HiCache] Yield idle scheduler so storage workers can drain (#38504)

test/registered/unit/managers/test_scheduler_on_idle_load.py

@@ -1,6 +1,6 @@
 """on_idle's stalled-path load publish is wall-clock bounded.
 
-A no-batch-but-not-idle stall spins on_idle without sleeping, so the gate must
+A non-storage no-batch stall spins on_idle without sleeping, so the gate must
 cap the O(queue) get_loads for both the DP-balancing writer and the load
 socket. CPU-only: builds a bare Scheduler with mocked collaborators, like
 test_scheduler_flush_cache.
@@ -23,6 +23,7 @@ class TestOnIdleStallPublish(CustomTestCase):
     def _stalled_scheduler(self) -> Scheduler:
         s = Scheduler.__new__(Scheduler)
         s.scheduler_stage_metrics = None
+        s.enable_hicache_storage = False
         s.maybe_send_health_check_signal = MagicMock()
         s.is_fully_idle = MagicMock(return_value=False)  # stalled, not idle
         s.publish_load_snapshot = MagicMock(return_value=None)