Add Atlas Cloud provider

[binyangzhu000-sudo]

Summary

• add an Atlas Cloud OpenAI-compatible provider with the default qwen/qwen3.5-flash model and https://api.atlascloud.ai/v1 endpoint
• register atlascloud in provider resolution, credential detection, judge selection, provider extras, and secret scrubbing
• document the Atlas Cloud setup path alongside the existing provider recipes

Validation

• uv run --python /Users/zby/.local/bin/python3.11 --with pytest --with pytest-asyncio --with openai --with pydantic --with typing_extensions --with pyyaml --with jsonschema --with click --with aiohttp --with python-dateutil --with json-repair --with rich --with questionary python -m pytest test_atlascloud_provider.py -q
• /Users/zby/.local/bin/python3.11 -m py_compile ifixai/providers/atlascloud.py ifixai/providers/resolver.py ifixai/providers/secrets.py test_atlascloud_provider.py
• uv run --python /Users/zby/.local/bin/python3.11 --with ruff ruff check ifixai/providers/atlascloud.py ifixai/providers/resolver.py ifixai/providers/secrets.py test_atlascloud_provider.py
• git diff --check
• verified Atlas Cloud live model catalogs include qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro