feat(router): 添加获取服务于所有用户的节点功能

修改 NodeRegistry 的 get_nodes_for_user 方法，除了获取显式映射到用户的节点外，现在还会包含服务于所有用户的节点（serves_users 为空的节点）。这确保了系统能正确处理全局可用的服务节点。

router/nodes.py

@@ -161,7 +161,17 @@ class NodeRegistry:
 
     def get_nodes_for_user(self, user_id: str) -> list[NodeConnection]:
         """Get all nodes that serve a user."""
-        node_ids = self._user_nodes.get(user_id, set())
+        # Get nodes explicitly mapped to this user
+        user_node_ids = self._user_nodes.get(user_id, set())
+        
+        # Get nodes that serve all users (empty serves_users set)
+        all_users_node_ids = set()
+        for node_id, node in self._nodes.items():
+            if not node.serves_users:
+                all_users_node_ids.add(node_id)
+        
+        # Combine both sets
+        node_ids = user_node_ids | all_users_node_ids
         return [self._nodes[nid] for nid in node_ids if nid in self._nodes]
 
     def get_active_node(self, user_id: str) -> Optional[NodeConnection]: