Automatically assign plugin-specific milestones

[zerlinpi]

Summary

Fixes #2541.

Automatically assigns a plugin-specific milestone when a pull request is opened or synchronized. The workflow counts additions and deletions under each known plugins/<slug>/ directory, selects the plugin with the unique highest total, and updates the pull request to that plugin's milestone.

It also adds focused unit tests to the existing JavaScript CI job.

Relevant technical choices

• The workflow runs from pull_request_target and checks out only the trusted base branch. It reads pull request file metadata through the GitHub API and never executes code from the contributor branch.
• Cross-plugin renames count additions for the destination plugin and deletions for the source plugin.
• An existing milestone is preserved when it already belongs to the selected plugin, allowing maintainers to choose a specific release manually.
• The generic <plugin> n.e.x.t milestone is preferred. A versioned milestone is assigned only when it is the plugin's sole open candidate.
• Ties and ambiguous milestone candidates leave the current milestone unchanged and emit an explanatory workflow message.
• The optional changelog-label enhancement described as a bonus in the issue is intentionally left for a separate change.

Use of AI Tools

I used OpenAI Codex to assist with repository research, implementation, and test drafting. I reviewed the resulting changes and validation output and take responsibility for the contribution.

Testing

• npm run test-unit-js
• npm run lint-js
• npm run tsc
• npm run lint-json

[github-actions]

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

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.35%. Comparing base (01a50eb) to head (4f11ba0).

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #2586   +/-   ##
=======================================
  Coverage   70.35%   70.35%           
=======================================
  Files          91       91           
  Lines        7867     7867           
=======================================
  Hits         5535     5535           
  Misses       2332     2332           

Flag	Coverage Δ
multisite	70.35% <ø> (ø)
single	35.17% <ø> (ø)

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