feat: add Atlas Cloud model config

[binyangzhu000-sudo]

Summary

• allow OpenAI-compatible model configs to load credentials from a provider-specific environment variable while preserving OPENAI_API_KEY as the default
• add an executable Atlas Cloud configuration for qwen/qwen3.8-max
• cover custom key environment resolution with a focused regression test

Testing

• python -m pytest tests/models/test_openai_api.py tests/models/test_openai_streaming.py -q (19 passed)
• python -m build --wheel --no-isolation
• OpenCompass live inference against https://api.atlascloud.ai/v1 with qwen/qwen3.8-max
• file-scoped flake8, isort, YAPF, codespell, quote, conflict, encoding, and line-ending hooks

The repository-wide dataset suffix hook was also attempted locally, but it cannot load two existing MathBench/WikiBench configs with the available mmengine runtime; no dataset files are changed by this PR.