Perf(test): shrink synthetic volumes in image loading and dataset tests

[aymuos15]

Description

Shrinks the synthetic volumes these six test files generate, so each test still exercises the same code paths in a fraction of the runtime. Pure test-fixture change: no production code touched, no test logic altered:

• tests/transforms/test_load_image.py, tests/transforms/test_load_imaged.py: 128³ volumes → 32³; multi-image cases 3×128³ → 3×32³; the 384×128³ channel-dim case scales to 96×32³.
• tests/data/test_arraydataset.py, test_cachedataset.py, test_dataset.py: 128³ → 32³; the Spacing case 64×64×33 → 16×16×9.
• tests/data/test_mapping_file.py: 128³ → 16³.

The 128³ sizes were an unexamined default; nothing depends on the magnitude, only that shapes round-trip through .nii.gz I/O. Expected shapes were updated in lockstep, so the assertions still verify the same behaviour.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

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ℹ️ Recent review info

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Run ID: b4636b2b-d466-4d24-8c7c-b5be6988ad6e

📥 Commits

Reviewing files that changed from the base of the PR and between 1f7bde9 and 21b5171.

📒 Files selected for processing (1)

• tests/transforms/test_load_image.py

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📝 Walkthrough

Walkthrough

The test suite now uses smaller image volumes in dataset and image-loading tests. Expected shapes and metadata fixtures were updated. The mapping-file test uses 16×16×16 volumes. Existing test coverage remains in place.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~8 minutes

Merge Risk: ⚪ Minimal · up to 21b51

The test fixtures use smaller volumes without an identified mismatch in the affected test operations. No actionable merge risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the performance-focused reduction of synthetic volumes in image-loading and dataset tests.
Description check	✅ Passed	The description explains the fixture changes, affected files, unchanged test logic, and change type. It omits the template's Fixes #... issue reference, but the required change details are otherwise…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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🧪 Generate unit tests (beta)

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