fix: add OpenAI adapter deregistration

[BrUn3y]

Pull Request

Issue

Fixes #1624

Description

Adds OpenAIBackend.remove_adapter() as the inverse of adapter registration.

• Removes registered embedded adapters and their cached composed-adapter configuration under the existing adapter lock.
• Clears the deprecated shim's backend reference and frees its qualified name for reuse.
• Treats removal of an unknown name as a logged no-op, matching the existing backend contract.
• Updates the adapter mixin documentation and contract tests.

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)

Verification performed locally:

• uv run pytest test/backends/test_adapters/test_adapter_mixin.py test/backends/test_openai_intrinsics_unit.py -m "not qualitative" — 43 passed.
• uv run pre-commit run --files ... — all applicable hooks passed.
• uv run pytest test/ -m "not qualitative" — 4,650 passed; 5 unrelated LiteLLM/Ollama tests failed because the local Ollama endpoint returned 404 for the unavailable granite4.2:3b model.

Attribution

• AI coding assistants used

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

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[jakelorocco]

thanks for the contribution! one small test change but looks good besides that

[BrUn3y]

@jakelorocco thanks for the comment, I've already applied your recommendation.