feat: add qwen provider

[oborges]

Summary

• add Alibaba Qwen provider using DashScope's OpenAI-compatible API
• register provider name qwen
• add llm-bridge[qwen] optional extra
• add offline fake-SDK tests and Qwen example
• document Qwen install, credentials, and endpoint behavior

Testing

• .venv/bin/ruff check .
• .venv/bin/black --check .
• .venv/bin/mypy src/llm_bridge
• .venv/bin/pytest

Live smoke test

Verified with qwen-plus using the international DashScope endpoint.

[github-actions]

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[oborges]

I have read the CLA Document and I hereby sign the CLA