feat: add Atlas Cloud backend helper

[binyangzhu000-sudo]

Pull Request

Issue

N/A

Description

Adds an Atlas Cloud OpenAI-compatible backend helper that reuses the existing OpenAIBackend path while applying Atlas Cloud defaults:

• adds AtlasCloudBackend and create_atlascloud_openai_backend
• reads ATLASCLOUD_API_KEY for auth, with optional ATLASCLOUD_API_BASE / ATLASCLOUD_BASE_URL overrides
• sets https://api.atlascloud.ai/v1 and qwen/qwen3.5-flash as defaults
• registers atlascloud, atlas-cloud, and atlas backend aliases for start_backend / start_session
• keeps provider metadata as atlascloud for telemetry paths

Testing

• Tests added to the respective file if code was changed
• New code has 100% coverage if code was added
• Ensure existing tests and github automation passes (a maintainer will kick off the github automation when the rest of the PR is populated)

Local validation:

• uv run ruff format mellea/backends/atlascloud.py mellea/stdlib/start_backend.py mellea/stdlib/session.py test/backends/test_atlascloud.py
• uv run pytest test/backends/test_atlascloud.py test/stdlib/test_session_unit.py -q
• uv run ruff check mellea/backends/atlascloud.py mellea/stdlib/start_backend.py mellea/stdlib/session.py test/backends/test_atlascloud.py test/stdlib/test_session_unit.py
• git diff --check
• uv run python -m py_compile mellea/backends/atlascloud.py mellea/stdlib/start_backend.py mellea/stdlib/session.py test/backends/test_atlascloud.py
• uv run mypy mellea/backends/atlascloud.py mellea/stdlib/start_backend.py mellea/stdlib/session.py test/backends/test_atlascloud.py
• uv run python tooling/docs-autogen/build.py
• uv run python tooling/docs-autogen/audit_coverage.py --docs-dir docs/docs/api --quality --fail-on-quality --threshold 100

The Atlas Cloud model catalog was checked live before committing; qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro were both present.

Attribution

• AI coding assistants used

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Adding a new component, requirement, sampling strategy, or tool?

• Component
• Requirement
• Sampling Strategy
• Tool

[planetf1]

@binyangzhu000-sudo thanks for the contribution.

Looking at the change you made to use Atlas Cloud as the backend, it seems as if this should already work w today with OpenAIBackend by passing base_url="https://api.atlascloud.ai/v1" and api_key=... directly (or via OPENAI_BASE_URL / OPENAI_API_KEY). I don't have access though to try this myself.

A new backend would make sense if it needs to implement new behaviour including there being limitations in the existing backends.

Was there a particular reason you looked at adding a backend - for extra function now? plans for the future? bugs in existing backend? poor docs?