Add Atlas Cloud provider catalog

[binyangzhu000-sudo]

Summary

• add Atlas Cloud as an OpenAI-compatible provider default using ATLASCLOUD_API_KEY and https://api.atlascloud.ai/v1
• register Atlas Cloud provider metadata, aliases, and two live-verified models in the builtin catalog
• add focused catalog and driver tests covering provider defaults, aliases, and model lookup

Validation

• cargo fmt --check
• cargo test -p openfang-runtime model_catalog::tests::test_atlascloud_provider_and_models_present
• cargo test -p openfang-runtime model_catalog::tests::test_catalog_has_providers
• cargo test -p openfang-runtime model_catalog::tests::test_new_providers_in_catalog
• cargo test -p openfang-runtime drivers::tests::test_provider_defaults_atlas
• cargo test -p openfang-runtime drivers::tests::test_known_providers_list
• git diff --check
• live Atlas Cloud /v1/models check returned 118 models and confirmed qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro

No root README changes.