Use modern image formats in background images for Cover blocks and Group blocks

[vipul0425]

Summary

Fixes #2111

Relevant technical choices

Adds functionality to process background images used in Cover and Group blocks, particularly for cases with special styling options like fixed or repetitive backgrounds. Our current approach was limited to only processing <img> tags in content, but this missed background images applied through inline CSS styles.

When users set specific styling options in Cover blocks (such as fixed backgrounds) or Group blocks with background images, these images are added via inline styles rather than standard <img> tags. As a result, these images weren't being converted to modern formats.

[github-actions]

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Co-authored-by: vipul0425 <vipulgupta003@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>

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[codecov]

Codecov Report

❌ Patch coverage is 86.36364% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.69%. Comparing base (42bb977) to head (ca2a61f).
⚠️ Report is 46 commits behind head on trunk.

Files with missing lines	Patch %	Lines
plugins/webp-uploads/hooks.php	90.00%	4 Missing ⚠️
plugins/webp-uploads/helper.php	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #2121      +/-   ##
==========================================
+ Coverage   67.22%   67.69%   +0.47%     
==========================================
  Files          93       93              
  Lines        7758     7845      +87     
==========================================
+ Hits         5215     5311      +96     
+ Misses       2543     2534       -9     

Flag	Coverage Δ
multisite	67.69% <86.36%> (+0.47%)	⬆️
single	36.30% <9.09%> (-0.41%)	⬇️

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[westonruter]

Thanks for working on this!

[westonruter]

This is getting close! The main thing left, other than the two minor suggestions I left, is to add PHPUnit tests for this new webp_uploads_filter_block_background_images() function.

[vipul0425]

Thanks @westonruter for the detailed review. I'll add the test cases and update the PR. 🙇

[westonruter]

Some micro nits to reduce extra newlines where they don't seem needed.

[westonruter]

Thank you!