feat(models): add Atlas Cloud chat provider

[binyangzhu000-sudo]

Summary

Adds Atlas Cloud as a chat provider — one entry in conf/model_providers.yaml, no code
change. It is OpenAI-compatible, so it follows the nebius/venice shape:

  atlascloud:
    name: Atlas Cloud
    litellm_provider: openai
    models_list:
      endpoint_url: "/models"
    kwargs:
      a0_api_mode: chat
      api_base: https://api.atlascloud.ai/v1

The provider id is atlascloud, so the key resolves from ATLASCLOUD_API_KEY (or
API_KEY_ATLASCLOUD) through the existing get_api_key() derivation in models.py — nothing else
needed. The relative models_list.endpoint_url lets the Model Configuration plugin populate the
dropdown instead of making users type model ids.

Only the chat section is touched. /v1/embeddings returns 404 on this endpoint, so listing
an embedding provider would be wrong.

Validation

Per CONTRIBUTING's "Include exact tests run, or clearly explain why validation was blocked":

• python -m pytest tests/test_model_config_ui.py -q → 7 passed
• tests/test_model_search.py and tests/test_model_config_api_keys.py could not run here:
  collection imports sentence_transformers, which pulls in torch, and installing that was out of
  scope for a one-line YAML change. Both only read this YAML, and the file still parses
  (yaml.safe_load) with the new entry — 30 chat providers resolved.
• The resolved model-listing URL (api_base + endpoint_url =
  https://api.atlascloud.ai/v1/models) returns 117 models over httpx, which is the client
  plugins/_model_config/api/model_search.py actually uses. (Worth noting: this endpoint's edge
  rejects Python's stdlib urllib default User-Agent with 403, but httpx and requests are
  fine — so the plugin's path works as-is.)
• The configured call path was exercised end to end with litellm using this exact YAML entry:
  litellm.completion(model="openai/deepseek-ai/deepseek-v4-pro", api_base=<kwargs.api_base>) →
  finish_reason=stop with content, 32 tokens.

---

🤝 Partnership & contact

This PR comes from the Atlas Cloud team. Beyond the provider entry above, we'd love to explore a closer collaboration with Agent Zero — for example co-marketing or a featured integration.

If that sounds interesting, reach out anytime:

• 📧 marketing@atlascloud.ai
• 🌐 https://www.atlascloud.ai
• 📚 Docs: https://www.atlascloud.ai/docs

And of course, happy to revise this PR to match your project's conventions — just leave a comment. 🙌