webp-uploads: Link to FAQ from Modern Image Formats settings section

[Gilmoursa]

Summary

Fixes #2442.

Adds a description paragraph to the Modern Image Formats settings section that links to the FAQ on WordPress.org, so users can find an explanation for why converted images may not appear after upload without leaving the admin dashboard.

Relevant technical choices

The FAQ in readme.txt already covers the two main reasons images aren't converted (converted file is larger than the original; image was not uploaded via the Media Library). Rather than duplicating that text, this links to it from the settings section.

__return_empty_string for the section callback is replaced with a named webp_uploads_modern_image_formats_section_callback() function (following the pattern used in speculation-rules/settings.php). A unit test is added to assert the output contains the FAQ URL and wrapper element.

Use of AI Tools

This PR was authored with Claude Code (Anthropic). Claude wrote the code and opened the PR. I reviewed the diff, the issue discussion, the PR template, and the CONTRIBUTING.md requirements before approving.

[github-actions]

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Co-authored-by: Gilmoursa <gilmoursa@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: b1ink0 <b1ink0@git.wordpress.org>

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[westonruter]

Hummm. You already opened a PR for this: #2480

Please use that existing pull request.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 69.37%. Comparing base (347597f) to head (89c151e).
⚠️ Report is 5 commits behind head on trunk.

Files with missing lines	Patch %	Lines
plugins/webp-uploads/settings.php	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #2483      +/-   ##
==========================================
+ Coverage   69.33%   69.37%   +0.03%     
==========================================
  Files          90       90              
  Lines        7749     7758       +9     
==========================================
+ Hits         5373     5382       +9     
  Misses       2376     2376              

Flag	Coverage Δ
multisite	69.37% <90.00%> (+0.03%)	⬆️
single	35.80% <90.00%> (+0.07%)	⬆️

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[Gilmoursa]

My apologies — I didn't realize #2480 was already open for this! Closing this one in favor of that. Thanks for catching it 🙏