Add Atlas Cloud LiteLLM provider alias

[binyangzhu000-sudo]

Summary

• add an atlascloud/ model alias that routes PageIndex LiteLLM calls through Atlas Cloud's OpenAI-compatible endpoint
• read ATLASCLOUD_API_KEY and optional ATLASCLOUD_API_BASE only when Atlas Cloud models are selected
• document a minimal Atlas Cloud config example and add focused unit coverage for the model mapping

Validation

• .venv/bin/python -m compileall pageindex
• .venv/bin/python -m pytest tests/test_atlascloud_litellm.py tests/test_issue_163.py
• git diff --check
• verified Atlas Cloud public model catalog contains qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro

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