[BugFix]fix backend ut (vllm-project#9805)

### What this PR does / why we need it?

This PR fixes the backend unit tests in
`tests/ut/distributed/ascend_store/test_backend.py`.
Recently, new attributes (such as `_lazy_init`, `_store_initialized`,
`_is_a2`, `_registered_buffers`, and `_buffers_registered`) were added
to `MooncakeBackend` and `MemcacheBackend` to support lazy
initialization and buffer registration.
Since the unit tests construct these backend instances using `__new__`
(bypassing `__init__`), these attributes were missing on the mocked
instances, causing unit test failures. This PR explicitly initializes
these attributes on the mocked backend instances to fix the tests.

- vLLM version: v0.20.2
- vLLM main: vllm-project/vllm@39910f2
---------
Signed-off-by: 房建伟 <fangjianwei@fangjianweideMacBook-Air.local>
Co-authored-by: 房建伟 <fangjianwei@fangjianweideMacBook-Air.local>

Author: DreamerLeader

tests/ut/distributed/ascend_store/test_backend.py

@@ -256,6 +256,9 @@ def _make_backend(self):
             backend = MooncakeBackend.__new__(MooncakeBackend)
             backend.store = MagicMock()
             backend.config = MagicMock()
+            backend._lazy_init = False
+            backend._store_initialized = True
+            backend._use_fabric_mem = False
             return backend
 
     def test_exists(self):
@@ -413,6 +416,12 @@ def _make_backend(self):
             backend = MemcacheBackend.__new__(MemcacheBackend)
             backend.store = MagicMock()
             backend.local_rank = 0
+            # Set internal state to avoid lazy init logic during tests
+            backend._lazy_init = False
+            backend._store_initialized = True
+            backend._is_a2 = False
+            backend._registered_buffers = None
+            backend._buffers_registered = False
             return backend
 
     def test_exists(self):