fix(test): unbreak test_kv_transfer_replica_metric after #35950 (#35974)

test/registered/unit/disaggregation/test_kv_transfer_replica_metric.py

@@ -45,9 +45,23 @@ def _make_kv_mgr(is_mla_backend):
     return mgr
 
 
+class _ConcreteKVSender(CommonKVSender):
+    """CommonKVSender is abstract: `poll` and `failure_exception` are backend
+    duties, so it cannot be instantiated (not even via __new__, which enforces
+    ABC completeness). This stub supplies them so the shared metric path can be
+    exercised without pulling in a transfer backend.
+    """
+
+    def poll(self):
+        raise NotImplementedError
+
+    def failure_exception(self):
+        raise NotImplementedError
+
+
 def _make_sender(kv_mgr):
     """CommonKVSender bypassing __init__, wiring only the fields the path reads."""
-    sender = CommonKVSender.__new__(CommonKVSender)
+    sender = _ConcreteKVSender.__new__(_ConcreteKVSender)
     sender._transfer_metric = KVTransferMetric()
     sender._transfer_num_kv_indices = 0
     sender._transfer_num_state_indices = 0