fix(gateway): preserve context-only run config

[xsfX20]

Summary

• keep DeerFlow context overrides inside config["context"] when the incoming request already uses LangGraph's context mode
• avoid reintroducing configurable in that path, which can violate LangGraph's context/configurable exclusivity
• add a regression test covering the context-only merge path

Testing

• git diff --check
• uv run pytest -q tests/test_gateway_services.py (dependency bootstrap started, but failed on a python-telegram-bot wheel download timeout in this environment)

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[xsfX20]

Closing this from my side as superseded by the current main branch. The gateway code now has merge_run_context_overrides(), which intentionally writes whitelisted context values to both config["configurable"] and config["context"] for the newer LangGraph runtime path. Rebasing this older patch would conflict with that current design and reintroduce the older context-only behavior.