fix: Handle check_model_inputs removal in transformers 5.2.0

[oliverholworthy]

What does this PR do ?

Fix get_check_model_inputs_decorator() to handle transformers >= 5.2.0 where check_model_inputs was removed and split into merge_with_config_defaults and capture_outputs, and fix the null_decorator fallback that was broken by @contextmanager.

Changelog

• Remove @contextmanager from null_decorator — it turned the function into a ContextDecorator whose __call__(self, func) signature collided with model forward kwargs like input_ids, causing TypeError at runtime.
• Add a new branch in get_check_model_inputs_decorator() for transformers >= 5.2.0 that composes merge_with_config_defaults and capture_outputs into a single decorator.
• Add tests verifying the fallback decorator works with keyword arguments and that null_decorator works in both @decorator and @decorator() forms.

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Additional Information

Affects all three models using get_check_model_inputs_decorator(): biencoder, llama, and qwen2. No model file changes needed, they already call the shared helper and will pick up the fix automatically.

[oliverholworthy]

/ok to test 7df5cd8