Add Atlas Cloud as a direct model provider

[binyangzhu000-sudo]

Summary

• add an atlas/<model-id> route for the Atlas Cloud OpenAI-compatible API
• resolve ATLASCLOUD_API_KEY and optional ATLASCLOUD_BASE_URL without sending Atlas requests through HF Router
• validate and probe Atlas model switches, and avoid requiring an HF token when local tools are used

Validation

• uv run --extra dev ruff check on all changed Python files
• uv run --extra dev pytest -q tests/unit --maxfail=1 (515 passed)
• uv run python -m compileall -q agent
• live deepseek-ai/deepseek-v4-pro forced tool-call smoke test through the new resolver

[lewtun]

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