Enhancement: reduce maximum image size used as background image based on container width

[nani-samireddy]

Summary

Fixes #2216

Relevant technical choices

This change exposes background image attachment IDs and original URLs from block attributes (core/group and core/cover) as data attributes on the rendered HTML element, and uses those attributes in Image Prioritizer to pick and serve a more appropriate-sized background image. It enables downsizing background images based on collected URL Metrics (preferring desktop metrics to avoid serving overly small images to desktop visitors).

Use of AI Tools

[codecov]

Codecov Report

❌ Patch coverage is 76.08696% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.91%. Comparing base (7938a41) to head (5ed9644).

Files with missing lines	Patch %	Lines
...rioritizer-background-image-styled-tag-visitor.php	70.00%	6 Missing ⚠️
...ns/auto-sizes/includes/improve-calculate-sizes.php	80.76%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #2258      +/-   ##
==========================================
+ Coverage   68.86%   68.91%   +0.04%     
==========================================
  Files          90       90              
  Lines        7610     7656      +46     
==========================================
+ Hits         5241     5276      +35     
- Misses       2369     2380      +11     

Flag	Coverage Δ
multisite	68.91% <76.08%> (+0.04%)	⬆️
single	35.54% <45.65%> (+0.06%)	⬆️

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[github-actions]

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Co-authored-by: nani-samireddy <[email protected]>
Co-authored-by: westonruter <[email protected]>

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