test: add Infection mutation testing with enforced score and dashboard reporting

[turegjorup]

Summary

Mirrors the mutation-testing setup from itk-dev/openid-connect-bundle#44–#51. Baseline: 198 mutants, 141 killed — Covered Code MSI 71% against 100% line coverage; the full run takes ~14 seconds. All 57 escaped mutants are in OpenIdConfigurationProvider (the library's one public class); follow-up PRs will kill them and ratchet the threshold.

Features Added

• Infection as a dev dependency with infection.json5 — minCoveredMsi: 68 and threads: max live in the config as the single source of truth (no flags duplicated across Taskfile/CI)
• task test:mutation, also appended to task pr:actions
• CI job Mutation tests (8.3, prefer-stable) — single-entry matrix names the PHP version; runs --logger-github for inline PR annotations; GITHUB_* env vars passed into the container for Infection's CI detection
• Stryker dashboard reporting for develop (uses the repo's STRYKER_DASHBOARD_API_KEY secret), feeding a new mutation score badge in the README
• README "Mutation Testing" section in the development docs

Files Changed

• composer.json - infection/infection dev dependency + allow-plugins entry (normalized)
• infection.json5 (new) - config incl. threshold and Stryker dashboard logger
• Taskfile.yml - test:mutation, added to pr:actions
• .github/workflows/php.yaml - mutation-tests job
• README.md - badge + Mutation Testing section
• CHANGELOG.md - Unreleased bullet
• .gitignore - generated infection.log/infection.html

Test Plan

• task test:mutation — passes locally (71% ≥ 68%), dashboard upload correctly skipped outside CI
• task lint:yaml / task lint:markdown — pass
• The new CI job should run on this PR; badge populates on first push to develop after merge

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f241f67) to head (feb3bc2).

Additional details and impacted files

@@             Coverage Diff             @@
##             develop       #49   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity        71        71           
===========================================
  Files              1         1           
  Lines            185       185           
===========================================
  Hits             185       185           

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

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