Add percent encoding of URLs in imagesrcset param of Link response header

[westonruter]

In testing Optimization Detective for the release in 2 days, I discovered a problem when I upload an image file with non-ASCII filename (البيسون.jpg):

It's returning a 404 because the image URLs in the imagesrcset aren't getting the same encoding as is the original URL in the href.

Link: <https://complicated-lyrebird-c797ca.instawp.xyz/wp-content/uploads/2025/02/%D8%A7%D9%84%D8%A8%D9%8A%D8%B3%D9%88%D9%86-1024x668.jpg>; rel="preload"; fetchpriority="high"; as="image"; imagesrcset="https://complicated-lyrebird-c797ca.instawp.xyz/wp-content/uploads/2025/02/البيسون-1024x668.jpg 1024w, https://complicated-lyrebird-c797ca.instawp.xyz/wp-content/uploads/2025/02/البيسون-300x196.jpg 300w, https://complicated-lyrebird-c797ca.instawp.xyz/wp-content/uploads/2025/02/البيسون-768x501.jpg 768w, https://complicated-lyrebird-c797ca.instawp.xyz/wp-content/uploads/2025/02/البيسون-1536x1002.jpg 1536w, https://complicated-lyrebird-c797ca.instawp.xyz/wp-content/uploads/2025/02/البيسون-2048x1336.jpg 2048w"; imagesizes="(782px < width) 644px, (max-width: 1024px) 100vw, 1024px"; media="screen and (782px < width)"

Expected versus actual:

- https://complicated-lyrebird-c797ca.instawp.xyz/wp-content/uploads/2025/02/%D8%A7%D9%84%D8%A8%D9%8A%D8%B3%D9%88%D9%86-1024x668.jpg
+ https://complicated-lyrebird-c797ca.instawp.xyz/wp-content/uploads/2025/02/البيسون-1024x668.jpg

This is something we missed in #1802 while fixing #1775.

So this PR adds URL encoding of the imagesrcset:

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Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: felixarntz <flixos90@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.70%. Comparing base (cb79463) to head (e731b4b).
Report is 2 commits behind head on release/3.9.0.

Additional details and impacted files

@@                Coverage Diff                @@
##           release/3.9.0    #1866      +/-   ##
=================================================
+ Coverage          66.64%   66.70%   +0.06%     
=================================================
  Files                 88       88              
  Lines               7015     7029      +14     
=================================================
+ Hits                4675     4689      +14     
  Misses              2340     2340              

Flag	Coverage Δ
multisite	66.70% <100.00%> (+0.06%)	⬆️
single	37.20% <0.00%> (-0.08%)	⬇️

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[westonruter]

Note: This was originally esc_url_raw( $encoded_url ?? '' ). However, this seems to have just been a way to work around the fact that preg_replace_callback() can technically return null when an invalid pattern is provided. However, since we know the pattern is correct based on tests, the use of (string) cast seems more appropriate.

[felixarntz]

@westonruter Great catch, LGTM!