fix: allow sglang-pinned OpenAI dependency

[binichallein]

Summary

• Fixes requirements.txt依赖版本冲突 #302 by relaxing the OpenAI dependency lower bound from >=2.9.0 to >=2.6.1 in both requirements.txt and setup.py.
• Keeps the package metadata and requirements file aligned.
• Adds a regression test that ensures the dependency spec allows the openai==2.6.1 version pinned by sglang 0.5.6.post1/post2.

Test Plan

• python -m pytest
• pre-commit run --files requirements.txt setup.py tests/test_dependency_constraints.py
• /tmp/open-autoglm-openai-261/bin/python -m pip install 'openai==2.6.1' 'Pillow>=12.0.0' 'requests>=2.31.0'
• /tmp/open-autoglm-openai-261/bin/python -m pip install -e . --no-deps
• Verified installed phone-agent metadata reports openai>=2.6.1 with openai version 2.6.1