test(nemotron_h): use a real NemotronHConfig in stateful-cache tests

[YHC66]

Description

TestNemotronHStatefulCache passes a bare MagicMock() as the HF model, so hf_model.config is a MagicMock too. On transformers 5.17, DynamicCache(config=...) goes through get_layer_types_and_kwargs, which does num_kv_shared_layers > 0, and both tests fail with:

TypeError: '>' not supported between instances of 'MagicMock' and 'int'

CI pins transformers 5.13.0, so this doesn't show there, but pyproject.toml allows transformers>=5.9.0 and the tests break as soon as the lock moves up.

The adapter itself is fine: with a real NemotronHConfig, DynamicCache(config=...) builds on both 5.13 and 5.17. This PR only changes the tests to hand create_stateful_cache a mock model carrying a tiny real NemotronHConfig (built with the typed layers_block_type argument, so mypy reports nothing new for the file).

Results:

	transformers 5.17	transformers 5.13 (locked)
test_nemotron_h_adapter.py before	2 failed, 43 passed	45 passed
test_nemotron_h_adapter.py after	45 passed	45 passed

On dev 02a7f5a0 with transformers 5.17, these two were the only failures in tests/unit -m "not slow" (6792 passed). With the locked environment, tests/unit -m "not slow" passes in full after the change (6898 passed).

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibility

[jlarson4]

Looks good, thanks @YHC66!