feat: add Atlas Cloud setup provider

[binyangzhu000-sudo]

Summary

• Add Atlas Cloud as a built-in /setup OpenAI-compatible provider preset.
• Label Atlas Cloud endpoints in the model/provider UI and backend error messages.
• Curate Atlas Cloud model discovery around live-verified chat and reasoning model IDs.

Linked Issue

• Related tracking: feat: add Atlas Cloud setup provider #5566

Type of Change

• New feature
• Bug fix
• Documentation update
• Refactor
• Test update

How to Test

Run the focused provider setup and endpoint tests:

.venv/bin/python -m pytest tests/test_slash_setup_provider_aliases.py tests/test_provider_endpoints_url_building.py tests/test_provider_classification.py tests/test_provider_label_js.py tests/test_provider_endpoints_headers.py tests/test_model_routes.py -q
/Users/zby/.local/bin/python3.11 -m py_compile src/llm_core.py routes/model_routes.py
node -e "import('./static/js/providers.js').then(m => console.log(m.providerLabel('https://api.atlascloud.ai/v1')))"
git diff --check

I also verified the live Atlas Cloud /v1/models catalog returns qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro.

Checklist

• I searched existing issues and PRs to avoid duplicates.
• I tested the changes locally.
• No README changes.