Gallery Block: Fix equal column gallery image sizes

[mukeshpanchal27]

Summary

Part of #2449

Relevant Technical Choices

Ensure that the number of images added matches the Gallery block's configured column count.

How to Test

1. Add a Gallery block.
2. Insert 3 images and make sure the Columns setting is set to 3.
3. Verify that the image sizes.
4. Repeat the test with 6 images while keeping the Columns setting at 3.
5. Verify that the image sizes.

Before

After

In follow-up PR will fix the sizes for other column settings.

• Add unit tests

Use of AI Tools

N/A

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.13%. Comparing base (b1608b2) to head (de04562).

Files with missing lines	Patch %	Lines
...ns/auto-sizes/includes/improve-calculate-sizes.php	62.50%	6 Missing ⚠️

Additional details and impacted files

@@                      Coverage Diff                       @@
##           feature/2449-gallery-block    #2534      +/-   ##
==============================================================
- Coverage                       69.14%   69.13%   -0.02%     
==============================================================
  Files                              90       90              
  Lines                            7723     7739      +16     
==============================================================
+ Hits                             5340     5350      +10     
- Misses                           2383     2389       +6     

Flag	Coverage Δ
multisite	69.13% <62.50%> (-0.02%)	⬇️
single	35.56% <62.50%> (+0.05%)	⬆️

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[mukeshpanchal27]

@westonruter I'd appreciate any initial feedback you can provide on the PR when you have a chance to take a look.

[adamsilverstein]

pr makes sense to me, it would be good to add some tests that demonstrate what it achieves.