* 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
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