Add Atlas Cloud YAML provider support

[binyangzhu000-sudo]

Summary

• Add atlascloud, atlas-cloud, and atlas provider aliases to the YAML model loader.
• Route Atlas Cloud through the existing OpenAI-compatible LocalModel path with default base URL https://api.atlascloud.ai/v1 and default model qwen/qwen3.5-flash.
• Read Atlas Cloud credentials and overrides from ATLASCLOUD_* and ATLAS_CLOUD_* environment variables, while preserving explicit YAML config precedence.
• Add focused tests for defaults, environment fallback, explicit overrides, and provider aliases.

Validation

• .venv/bin/python -m pytest tests/test_core/test_yaml/test_atlascloud_model_callback.py -q (4 passed)
• .venv/bin/python -m pytest tests/test_core/test_yaml/test_yaml.py::test_target_loads_from_yaml_callback tests/test_core/test_yaml/test_yaml.py::test_models_loads_from_yaml_class -q (2 passed)
• .venv/bin/python -m black --check deepteam/cli/model_callback.py tests/test_core/test_yaml/test_atlascloud_model_callback.py
• PYTHONPYCACHEPREFIX=/tmp/deepteam-pyc .venv/bin/python -m compileall deepteam/cli/model_callback.py tests/test_core/test_yaml/test_atlascloud_model_callback.py
• git diff --check
• Atlas Cloud catalog check returned 436 models and confirmed qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro.

Note: the broader tests/test_core/test_yaml/test_yaml.py run still needs an OPENAI_API_KEY for the existing default OpenAI model construction in two tests; the non-OpenAI-key YAML tests above pass.

No README changes; the only example change is a small YAML provider snippet. No logo, sponsor, credits, or partner content was added.

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