Add test comparing WebP and JPEG sub-size dimensions

[sonali195]

Summary

Adds a PHPUnit test checking that each WebP sub-size has the same width and
height as the matching JPEG sub-size in the attachment metadata, using an image
large enough to be scaled.

Fixes #ISSUE_NUMBER

How to test

1. Run npm run wp-env-test start.
2. Run npm run test-php -- --filter test_it_should_create_webp_subsizes_with_the_same_dimensions.

Use of AI tools

I used Claude to help draft the test. I reviewed the code and ran the test myself.

[github-actions]

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Co-authored-by: sonali195 <sonaliprajapati@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.50%. Comparing base (a25f804) to head (9802832).

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #2696   +/-   ##
=======================================
  Coverage   70.50%   70.50%           
=======================================
  Files          91       91           
  Lines        7869     7869           
=======================================
  Hits         5548     5548           
  Misses       2321     2321           

Flag	Coverage Δ
multisite	70.50% <ø> (ø)
single	35.34% <ø> (ø)

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