Add Atlas Cloud provider

[binyangzhu000-sudo]

Summary

• add a dedicated providers/atlascloud package that reuses the existing OpenAI-compatible provider implementation
• set Atlas Cloud's default LLM API base URL to https://api.atlascloud.ai/v1
• add focused provider tests and include Atlas Cloud provider options in the registry round-trip coverage

Validation

• verified qwen/qwen3.5-flash is present in the live Atlas Cloud model list with display_console: true
• git diff --check
• self-reviewed the staged diff

I could not run go test locally because this environment does not have the Go toolchain (go/gofmt) installed.

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

[andreynering]

Hey @binyangzhu000-sudo, thanks for wanting to contribute.

But the truth is that this should not be needed at all. Most providers are use /chat/completions and are well served by using just the OpenAI-compat provider.