Fix: Skip scaled image sideload for images below big image threshold

[adamsilverstein]

Summary

Adds a dimension check before queuing the scaled image sideload, so that images below the big image size threshold (default 2560px) are not incorrectly processed as scaled images.

Problem

After #75817 ("Fix client-side media file naming"), the generateThumbnails() function unconditionally queues a "scaled" sideload for all images when bigImageSizeThreshold is set, regardless of actual image dimensions. This causes the PHP handler to set metadata['original_image'] even for small images that don't need scaling.

This breaks the big-image-size-threshold.spec.js E2E test at line 302, which correctly expects original_image to be undefined for undersized images.

Root cause chain:

1. packages/upload-media/src/store/private-actions.ts — if ( bigImageSizeThreshold && attachment.id ) has no dimension check
2. A sideload with image_size: 'scaled' is sent to the server even for small images
3. The PHP handler sees image_size === 'scaled' and unconditionally sets metadata['original_image']

Fix

Uses createImageBitmap() to check the source file dimensions before queuing the scaled sideload. The sideload is only queued when width or height exceeds bigImageSizeThreshold, matching WordPress core's wp_create_image_subsizes() behavior.

Test coverage

Adds 8 unit tests for generateThumbnails in packages/upload-media/src/store/test/actions.ts, which previously had no test coverage. The tests verify:

• Images below the threshold do not get a scaled sideload
• Images above the threshold do get a scaled sideload with the correct attachment ID
• Images where only the height exceeds the threshold are scaled (width OR height check)
• No scaling when bigImageSizeThreshold is not set in settings
• No scaling when the attachment has no ID
• Sideload items are created for each missing image size
• Early return when the item has no attachment
• Images exactly at the threshold are not scaled (> not >=, matching WP core)

These tests were verified to fail against the pre-fix code, confirming they catch the bug.

Testing instructions

• Run npm run test:unit -- --testPathPattern="packages/upload-media/src/store/test/actions" — all 21 tests pass
• Run npm run test:e2e -- test/e2e/specs/editor/various/big-image-size-threshold.spec.js
• Confirm all 3 E2E tests pass (above threshold scaled, below threshold not scaled, custom naming)
• Upload a small image (below 2560px) and verify no original_image in attachment metadata
• Upload a large image (above 2560px) and verify -scaled version is created correctly AND original_image in attachment metadata

Screenshots

Baseline server processing

Before fix, note extra file

After fix

With large image

Scaled still working

[github-actions]

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Co-authored-by: adamsilverstein <adamsilverstein@git.wordpress.org>

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[github-actions]

I just cherry-picked this PR to the wp/7.0 branch to get it included in the next release: 9e6d6fa