[Feature] Add Atlas Cloud as a built-in model provider

### Feature Summary

Add Atlas Cloud as a built-in OpenAI chat.completions-compatible provider.

### Component

Model Providers / Provider Management

### Problem Statement

Atlas Cloud users can configure it as a custom OpenAI-compatible endpoint today, but QwenPaw does not provide a discoverable built-in preset with the correct base URL and tested model IDs.

### Proposed Solution

- Register an `OpenAIProvider` preset using `https://api.atlascloud.ai/v1`.
- Seed a small set of verified text models.
- Enable model discovery through the existing `/models` path.
- Add focused registration tests and list the provider in the model documentation.

This reuses the existing OpenAI-compatible runtime and does not add provider-specific transport logic.

### Alternatives Considered

Users can continue adding a custom provider manually, but that loses the built-in discovery and configuration experience.

### Additional Context

Atlas Cloud exposes an OpenAI-compatible model catalog and chat completions API. I am willing to implement and test this contribution.