Add Atlas Cloud provider

[binyangzhu000-sudo]

Summary

• add an Atlas Cloud provider that reuses the existing OpenAI-compatible provider path
• register atlascloud in the provider registry and LiteLLM prefix mapping
• document the provider dependency row and add focused provider tests

Validation

• python3 -m pytest -o addopts='' tests/test_atlascloud.py tests/test_agent_module_callable.py -q
• python3 -m compileall cascadeflow/providers/atlascloud.py cascadeflow/providers/__init__.py cascadeflow/providers/base.py tests/test_atlascloud.py
• git diff --check
• Atlas live catalog returned qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro

Notes: local environment does not have ruff or black installed, so those checks could not be run here.

README: no README changes; docs update only, no sponsor/logo/credits/partner promotion.

[saschabuehrle]

Thanks for the compact OpenAI-compatible provider implementation. The endpoint matches Atlas Cloud’s current official docs and the isolated tests, Ruff, and Black pass, but two runtime integration blockers remain.

Required before merge:

1. Register atlascloud in ModelConfig.validate_provider and PROVIDER_CAPABILITIES. Today ModelConfig(name="x", provider="atlascloud", cost=0) raises a validation error, so a CascadeAgent cannot configure it.
2. Fix provider attribution inherited from OpenAIProvider. A mocked successful request correctly calls https://api.atlascloud.ai/v1/chat/completions but returns ModelResponse.provider == "openai"; inherited errors also identify OpenAI. This breaks provider telemetry and routing traces.
3. Fix or explicitly define cost behavior. Current _get_litellm_prefix() produces openai/qwen/qwen3.5-flash, which LiteLLM rejects before falling back to inherited OpenAI estimates. Add Atlas-aware estimation/capability semantics and tests.
4. Add an end-to-end provider test covering ModelConfig/registry resolution plus response attribution, not only constructor registration.

Please update against current main and re-request review.