feat(models): add Atlas Cloud provider

[binyangzhu000-sudo]

Summary

• add Atlas Cloud as an optional native model provider using its OpenAI-compatible chat completions endpoint
• register a dedicated config and alias while keeping existing defaults unchanged
• document environment setup and add focused registry, client, credential, and token-accounting tests

Validation

• pytest -q tests/src/models/test_atlascloud_model.py (4 passed)
• pytest -q tests/src/models/test_openai_model.py tests/src/models/test_openrouter_model.py (36 passed)
• ruff check and ruff format --check on new provider and test files
• python -m compileall -q evoagentx/models
• one live chat-completions smoke with openai/gpt-5.6-luna