Missing resampling use case in tests

[oesteban]

Antecedent: https://github.com/nipy/nitransforms/pull/278#discussion_r2276640808

As per codex:

Input spatialimage	Transform	Covered?	Tests exercising the case
3D (incl. 4D singleton)	3D	✅ Yes	test_apply_singleton_time_dimension, test_apply_linear_transform, test_displacements_field1, test_displacements_field2, test_apply_transformchain
4D	4D	✅ Yes	test_LinearTransformsMapping_apply (functional realignment), test_apply_4d (synthetic per-volume transforms)
3D (incl. 4D singleton)	4D	✅ Yes	test_LinearTransformsMapping_apply (moving fieldmap with head-motion transforms)
4D	3D	❌ No	No test applies a single 3D transform across multiple timepoints of a 4D dataset (aside from the singleton-time regression check)