[BugFix] Fix multi async save in MultiConnector (#18246)

Signed-off-by: Nick Hill <nhill@redhat.com>

vllm/distributed/kv_transfer/kv_connector/v1/multi_connector.py

 # SPDX-License-Identifier: Apache-2.0
 import copy
+from dataclasses import dataclass
 from typing import TYPE_CHECKING, Any, Optional
 
 import torch
@@ -21,9 +22,10 @@ if TYPE_CHECKING:
 logger = init_logger(__name__)
 
 
-class MultiKVConnectorMetadata(tuple[KVConnectorMetadata, ...],
-                               KVConnectorMetadata):
-    pass
+@dataclass
+class MultiKVConnectorMetadata(KVConnectorMetadata):
+    metadata: tuple[KVConnectorMetadata, ...]
+    extra_async_saves: Optional[dict[str, int]] = None
 
 
 class MultiConnector(KVConnectorBase_V1):
@@ -54,6 +56,7 @@ class MultiConnector(KVConnectorBase_V1):
         # Keeps track of *additional* remaining async saves (beyond 1) to be
         # finished per request. Not needed for async loads since we only allow
         # a single connector to load.
+        # Propagated from scheduler to worker side via the connector metadata.
         self._extra_async_saves: dict[str, int] = {}
 
     def register_kv_caches(self, kv_caches: dict[str, torch.Tensor]):
@@ -66,7 +69,10 @@ class MultiConnector(KVConnectorBase_V1):
     def bind_connector_metadata(
             self, connector_metadata: KVConnectorMetadata) -> None:
         assert isinstance(connector_metadata, MultiKVConnectorMetadata)
-        for c, cm in zip(self._connectors, connector_metadata):
+        if connector_metadata.extra_async_saves:
+            self._extra_async_saves.update(
+                connector_metadata.extra_async_saves)
+        for c, cm in zip(self._connectors, connector_metadata.metadata):
             c.bind_connector_metadata(cm)
 
     def clear_connector_metadata(self) -> None:
@@ -152,8 +158,13 @@ class MultiConnector(KVConnectorBase_V1):
     def build_connector_meta(
             self,
             scheduler_output: SchedulerOutput) -> MultiKVConnectorMetadata:
-        return MultiKVConnectorMetadata(
-            c.build_connector_meta(scheduler_output) for c in self._connectors)
+        metadata = MultiKVConnectorMetadata(metadata=tuple(
+            c.build_connector_meta(scheduler_output)
+            for c in self._connectors))
+        if self._extra_async_saves:
+            metadata.extra_async_saves = self._extra_async_saves
+            self._extra_async_saves = {}
+        return metadata
 
     def request_finished(
         self,