fix(greptile): drain background refresh + warn on router mode override

Address the two new findings from greptile's 19:45 review of the
vertex+router surfaces.

- vertex_llm_base: when the slow path sees TokenState.INVALID, await any
  in-flight background refresh task before invoking refresh_auth
  ourselves. google-auth's Credentials.refresh() is not safe to call
  concurrently on the same credentials object, and the background task
  runs outside the per-key lock. After the wait, re-check the cached
  token so we can short-circuit if the background refresh already
  restored it. Extracted the helper into
  _await_in_flight_background_refresh so get_access_token_async stays
  under ruff's PLR0915 statement budget.
- router.py: when alias registration would overwrite the deployment's
  declared `mode` to keep the shared backend mode stable, emit a
  verbose_router_logger.warning so the override is visible to operators
  instead of silently winning. The existing fix (preventing alias
  registration from downgrading a shared `mode: responses` to chat) is
  preserved; the warning just surfaces it.

litellm/llms/vertex_ai/vertex_llm_base.py

@@ -500,6 +500,27 @@ class VertexBase:
                 exc_info=True,
             )
 
+    async def _await_in_flight_background_refresh(
+        self, credential_cache_key: tuple
+    ) -> None:
+        """Wait for an in-flight background refresh to finish, if any.
+
+        google-auth's ``Credentials.refresh()`` is not safe to invoke
+        concurrently on the same credentials object. Coroutines that need a
+        blocking refresh must first drain any background refresh that was
+        scheduled while a previous STALE token was being served.
+        """
+        existing_task = self._background_refresh_tasks.get(credential_cache_key)
+        if existing_task is None or existing_task.done():
+            return
+        try:
+            await existing_task
+        except Exception:
+            # Background refresh failures are already logged inside
+            # _background_refresh_credentials; the caller will fall through
+            # to its own blocking refresh.
+            pass
+
     def _schedule_background_refresh(
         self,
         credentials: Any,
@@ -1021,6 +1042,20 @@ class VertexBase:
                         return current_token, resolved_project
 
                 if token_state == TokenState.INVALID:
+                    # Drain any in-flight background refresh before invoking
+                    # refresh_auth ourselves; google-auth's
+                    # Credentials.refresh() is not safe to call concurrently
+                    # on the same credentials object, and the background task
+                    # runs outside this lock.
+                    await self._await_in_flight_background_refresh(
+                        credential_cache_key
+                    )
+                    cached = self._try_get_cached_token(
+                        credential_cache_key, project_id
+                    )
+                    if cached is not None:
+                        return cached
+
                     # Token is expired or missing — must block until refresh completes.
                     try:
                         verbose_logger.debug("Credentials expired, refreshing")

litellm/router.py

@@ -7331,6 +7331,17 @@ class Router:
                 # Multiple aliases can point at the same provider/model backend,
                 # but their deployment-level overrides should not downgrade the
                 # backend from responses -> chat via last-write-wins registration.
+                _deployment_mode = _shared_model_info.get("mode")
+                if _deployment_mode is not None:
+                    verbose_router_logger.warning(
+                        "Router: preserving existing mode=%s for shared backend "
+                        "key %s instead of the deployment-specified mode=%s "
+                        "(prevents alias registration from downgrading the "
+                        "shared backend mode).",
+                        _existing_shared_mode,
+                        _model_name,
+                        _deployment_mode,
+                    )
                 _shared_model_info["mode"] = _existing_shared_mode
             _backend_alias_cost = {_model_name: _shared_model_info}
             if "responses/" in _model_name: