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: 0ea39420-f440-41f2-85fe-bb02cfa775b5

📥 Commits

Reviewing files that changed from the base of the PR and between 8690ae7 and 1f7bde9.

📒 Files selected for processing (6)

• tests/data/test_arraydataset.py
• tests/data/test_cachedataset.py
• tests/data/test_dataset.py
• tests/data/test_mapping_file.py
• tests/transforms/test_load_image.py
• tests/transforms/test_load_imaged.py

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📝 Walkthrough

Walkthrough

The test suite now generates smaller 32³ image volumes for dataset and image-loading tests. Expected shapes and metadata fixtures were updated accordingly. The mapping-file test uses 16³ volumes. Coverage for transforms, labels, metadata, readers, channel dimensions, caching, and DataLoader batching remains unchanged.

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

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the performance-focused reduction of synthetic test volumes.
Description check	✅ Passed	The description explains the fixture changes, affected files, preserved behavior, and applicable change type.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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