Add Atlas Cloud model provider

[binyangzhu000-sudo]

Summary

• add an Atlas Cloud OpenAI-compatible model client using ATLASCLOUD_API_KEY / ATLAS_CLOUD_API_KEY and Atlas base URL aliases
• register atlascloud, atlas-cloud, and atlas routing prefixes in the model factory and expose --platform atlascloud
• add focused offline tests for config aliases, provider detection, model id normalization, and OpenAI-compatible client initialization

Validation

• python3 -m unittest tests.test_atlascloud_provider
• PYTHONPYCACHEPREFIX=/tmp/mingli-bench-atlas-pycache python3 -m compileall mingli_bench tests
• UV_CACHE_DIR=/tmp/uv-cache-mingli-bench UV_TOOL_DIR=/tmp/uv-tools-mingli-bench uvx --from ruff ruff check mingli_bench/models/atlascloud_client.py mingli_bench/models/factory.py mingli_bench/utils/config.py mingli_bench/cli.py tests/test_atlascloud_provider.py
• python3 -m mingli_bench.cli --list-models
• git diff --check
• Atlas live catalog returned 436 models and confirmed qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro

Notes

• No README, docs, logo, sponsor, credits, or partner copy changes.
• .env.example only adds optional technical configuration for the new provider.