fix(autogen-ext): fix __unknown_models set difference and outdated docstrings (#8243)

[Manohar Paturi (ManoharPaturi)]

Description

Fixes #8243.

1. In _message_transform.py, the previous assignment to __unknown_models accidentally overwrote the set difference calculation and failed to subtract __llama_models, leaving dead code in the initial assignment. This PR properly subtracts all known model families (__openai_models, __claude_models, __gemini_models, __llama_models, and __mistral_models).
2. Updated the docstring example in _transformation/registry.py where register_transformer was missing the api argument.
3. Updated the docstring example in _message_transform.py to reflect the current get_transformer API and return type.
4. Added unit test test_unknown_models_excludes_known_families in test_openai_model_client.py.

Checklist

• Tests pass
• Lint passes (ruff check)
• Signed-off commit with DCO

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