feat: add Atlas Cloud LLM provider

[binyangzhu000-sudo]

What does this PR do?

Adds Atlas Cloud as an OpenAI-compatible LLM provider for LEANN.

• Adds Atlas Cloud runtime settings with ATLASCLOUD_API_KEY / ATLAS_CLOUD_API_KEY and base URL aliases, defaulting to https://api.atlascloud.ai/v1.
• Adds AtlasCloudChat and get_llm support for atlascloud, atlas-cloud, and atlas provider aliases.
• Wires the CLI ask / react paths and the base RAG example to resolve Atlas Cloud API key, base URL, and default model (deepseek-ai/deepseek-v4-pro).
• Adds focused offline tests for settings resolution, OpenAI-compatible client initialization, chat completion request parameters, error handling, and factory aliases.

No README, docs, logo, credits, sponsor, or partner content is changed.

Related Issues

Fixes #

Validation

• uvx ruff format packages/leann-core/src/leann/settings.py packages/leann-core/src/leann/chat.py packages/leann-core/src/leann/cli.py apps/base_rag_example.py tests/test_atlascloud_provider.py
• uvx ruff check packages/leann-core/src/leann/settings.py packages/leann-core/src/leann/chat.py packages/leann-core/src/leann/cli.py apps/base_rag_example.py tests/test_atlascloud_provider.py
• python3 -m pytest tests/test_atlascloud_provider.py tests/test_minimax_provider.py tests/test_novita_provider.py -q (60 passed, 7 skipped)
• python3 -m compileall packages/leann-core/src/leann apps/base_rag_example.py tests/test_atlascloud_provider.py
• git diff --check && git diff --cached --check
• Confirmed default model/base URL against the local Atlas API reference (/Users/zby/opensource/api.md): https://api.atlascloud.ai/v1, deepseek-ai/deepseek-v4-pro, deepseek-ai/deepseek-v4-flash, qwen/qwen3.5-27b.

Local environment notes:

• uv run pytest / uv run ruff ... could not start in this checkout because packages/astchunk-leann is present as an empty directory, so uv fails metadata generation before running commands.
• pre-commit is not installed in the local PATH.
• Atlas live /models catalog requests returned HTTP 403 from this machine, so no paid chat-completions live test was run.

Checklist

• Tests pass (focused provider suite via python3 -m pytest)
• Code formatted (uvx ruff format and uvx ruff check)
• Pre-commit hooks pass (pre-commit run --all-files unavailable locally)