feat(client): support custom middleware injection (#1520)

* feat(client): support custom middleware injection

Add support for custom middleware, allowing custom middleware list to be passed when initializing DeerFlowClient. These middleware will be injected after the default middleware when creating the agent, extending the agent's functionality.

* feat: inject custom middlewares before ClarificationMiddleware to preserve ordering

- Add `custom_middlewares` param to `_build_middlewares`
- Inject custom middlewares right before `ClarificationMiddleware` to keep it as the last in the chain
- Remove unsafe `.extend()` in `client.py`
- Update tests in `test_client.py` and `test_lead_agent_model_resolution.py` to assert correct injection ordering

backend/tests/test_lead_agent_model_resolution.py

@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+from unittest.mock import MagicMock
+
 import pytest
 
 from deerflow.agents.lead_agent import agent as lead_agent_module
@@ -133,9 +135,13 @@ def test_build_middlewares_uses_resolved_model_name_for_vision(monkeypatch):
     middlewares = lead_agent_module._build_middlewares(
         {"configurable": {"model_name": "stale-model", "is_plan_mode": False, "subagent_enabled": False}},
         model_name="vision-model",
+        custom_middlewares=[MagicMock()]
     )
 
     assert any(isinstance(m, lead_agent_module.ViewImageMiddleware) for m in middlewares)
+    # verify the custom middleware is injected correctly
+    assert len(middlewares) > 0 and isinstance(middlewares[-2], MagicMock)
+
 
 
 def test_create_summarization_middleware_uses_configured_model_alias(monkeypatch):