Add Atlas Cloud to the built-in model catalog

[binyangzhu000-sudo]

Summary

• add Atlas Cloud as a built-in OpenAI-compatible model provider
• wire ATLASCLOUD_API_KEY, https://api.atlascloud.ai/v1, and two live-confirmed Atlas model IDs into the model catalog
• extend focused catalog tests for the provider, auth env, base URL, adapter, and reasoning model entry

Validation

• python3 -m json.tool src/loushang/ai/model/models.json
• uv run --with pytest --with ruff pytest tests/ai/test_model_catalog.py tests/ai/test_model_loader_schema.py tests/ai/test_model_registry.py -q (53 passed)
• uv run --with ruff ruff check tests/ai/test_model_catalog.py
• git diff --check
• Atlas Cloud live /v1/models catalog check confirmed qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro

No README changes. No sponsor, logo, credits, or partner promotion changes.