Fix affine-elastic composition order

[ShreyanshGoyal]

Fixes #1491.

Description

Correct the sampling-grid composition for combined affine and elastic spatial transforms. Resampling evaluates the inverse transform, so an affine-then-elastic forward transform must undo the elastic displacement before applying the affine output-to-input mapping. The previous branches implemented the opposite order.

Add an analytical regression test using a 2x affine scale and a constant one-voxel displacement. These non-commuting transforms make the expected coordinates explicit for both affine_first values.

Validation:

• pytest -q tests/test_spatial.py (112 passed)
• ruff check src/torchio/transforms/spatial/spatial.py tests/test_spatial.py
• ruff format --check src/torchio/transforms/spatial/spatial.py tests/test_spatial.py
• The new regression test fails with the production source reverted to main and passes with this fix.

Checklist

• I have read the CONTRIBUTING docs and have a developer setup ready
• Changes are
  • Non-breaking (would not break existing functionality)
  • Breaking (would cause existing functionality to change)
• Tests added or modified to cover the changes
• In-line docstrings updated
• Documentation updated
• This pull request is ready to be reviewed