Fix/tidy and gd tests

[nickchomey]

Summary

Fixes #2524
Depends on #2523 (I cant be bothered to rebase back to trunk, and then back on top when that gets merged)

Relevant technical choices

I noticed some strange/redundant code in the dominant-color-images plugin so took it as an opportunity to do a bit of tidying up as well as implement tests for GD.

The commits are self-describing. Let me know if it is necessary to split this into a few PRs...

Use of AI Tools

I used Deepseek V4 via VS Code Github Copilot Chat to assist me. I reviewed and refactored its output.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 85.18519% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.27%. Comparing base (98437a5) to head (30c680c).
⚠️ Report is 2 commits behind head on trunk.

Files with missing lines	Patch %	Lines
plugins/dominant-color-images/helper.php	78.94%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #2527      +/-   ##
==========================================
+ Coverage   70.10%   70.27%   +0.17%     
==========================================
  Files          91       91              
  Lines        7823     7818       -5     
==========================================
+ Hits         5484     5494      +10     
+ Misses       2339     2324      -15     

Flag	Coverage Δ
multisite	70.27% <85.18%> (+0.17%)	⬆️
single	35.85% <85.18%> (+0.32%)	⬆️

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