Add unit tests for diffusion model eval branch in eval_cli.py

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Description

The diffusion-model eval branch (is_diffusion_model → diffusion_load_model → evaluate_diffusion_model) in eval_cli.eval() had no test coverage. Adds lightweight unit tests that monkeypatch these functions to verify the branch is taken correctly without requiring diffusers or model downloads.

New test class TestEvalDiffusionModelBranch in test/test_cpu/advanced/test_evaluation_functions.py:

• test_diffusion_branch_taken — patches is_diffusion_model to return True, asserts diffusion_load_model is called with the model path and evaluate_diffusion_model is called with (args, pipe=pipe)
• test_non_diffusion_model_skips_branch — patches is_diffusion_model to return False, asserts evaluate_diffusion_model is never invoked

with (
    patch("auto_round.eval.eval_cli.is_diffusion_model", return_value=True),
    patch("auto_round.utils.diffusion_load_model", return_value=(mock_pipe, MagicMock())) as mock_load,
    patch("auto_round.eval.evaluation.evaluate_diffusion_model") as mock_eval,
):
    eval_fn(args)

mock_load.assert_called_once_with(args.model)
mock_eval.assert_called_once_with(args, pipe=mock_pipe)

Type of Change

• Bug fix
• New feature
• Documentation update
• Performance improvement
• Code refactoring
• Other (please specify): Test coverage

Related Issues

Relates to #1522

Checklist Before Submitting

• My code has been tested locally.
• Documentation has been updated as needed.
• New or updated tests are included where applicable.

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