Add Atlas Cloud provider preset

[binyangzhu000-sudo]

Summary

• Add Atlas Cloud to the setup and /config provider preset catalog as an OpenAI-compatible provider.
• Prefill the Atlas Cloud base URL, default model, and model candidates for qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro.
• Add focused tests for the provider preset/defaults and update the bundled config example.

Validation

• git diff --check
• Confirmed config.example.jsonc matches internal/bootstrap/config.example.jsonc
• Confirmed Atlas Cloud /v1/models returns both qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro
• Go tests were not run locally because go is not installed in this environment.

README / promotion compliance

• No README changes.
• No sponsor, logo, credits, or partner content added.