ci: name the PHP version and dependency set in the mutation-tests job

[turegjorup]

Summary

The mutation-tests job ran on the default phpfpm service without saying which PHP version that is. It now declares PHP version and dependency set via a single-entry matrix, mirroring the unit-tests job, so the check name reads Mutation tests (8.3, prefer-stable) in the PR checks list.

Files Changed

• .github/workflows/php.yaml - single-entry matrix (service/php/prefer) on the mutation-tests job; the compose service is selected via ${{ matrix.service }} instead of being hard-coded
• CHANGELOG.md - Unreleased bullet

Test Plan

• The renamed check on this PR should appear as Mutation tests (8.3, prefer-stable) and pass
• prettier (yaml) + markdownlint — clean

Notes

• Branch protection/required checks: if Mutation tests is ever added as a required status check by its old name, the rename must be reflected there
• Extending mutation testing to more PHP versions later is now a one-line matrix addition

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (5e74d38) to head (758845b).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##             develop       #50   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity        62        62           
===========================================
  Files              9         9           
  Lines            282       282           
===========================================
  Hits             282       282           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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