fix: keep lead agent graph factory signature compatible

[greatmengqi]

Summary

• restore the public lead-agent graph factory to the LangGraph-compatible make_lead_agent(config) signature
• move the implementation to internal _make_lead_agent(config, *, app_config) so non-LangGraph callers can bind an explicit AppConfig without changing the public graph ABI
• add regression tests for both the public graph factory signature and explicit internal app-config injection

Context

Fixes the LangGraph Server load failure reported on #2612: #2612 (comment)

Validation

• uv run pytest -q tests/test_lead_agent_model_resolution.py tests/test_run_worker_rollback.py
• uv run ruff check packages/harness/deerflow/agents/lead_agent/agent.py tests/test_lead_agent_model_resolution.py

[Copilot]

Pull request overview

Restores the public lead-agent graph factory to the LangGraph Server–compatible make_lead_agent(config) ABI while still enabling non-LangGraph callers to inject an explicit AppConfig via an internal implementation.

Changes:

• Reverted make_lead_agent to a single-argument config signature and delegated to an internal _make_lead_agent(..., *, app_config).
• Added regression tests to lock the public factory signature and ensure explicit AppConfig injection avoids ambient get_app_config() reads.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
backend/packages/harness/deerflow/agents/lead_agent/agent.py	Restores LangGraph-compatible factory signature and moves injectable implementation behind _make_lead_agent.
backend/tests/test_lead_agent_model_resolution.py	Adds tests for public factory signature and explicit AppConfig injection behavior.