ci: run mutation tests with Infection

[roxblnfk]

What was changed

• Mutation Tests workflow runs Infection on PRs and pushes to 2.x (PHP 8.4, Xdebug) with --min-msi=85 --min-covered-msi=85; pushes to 2.x send the report to the Stryker dashboard, and the README gets its badge.
• Local run: 39 mutants, 35 killed, MSI 89%, mutation code coverage 100%. Escaped: three boundary flips in the assert() argument checks of create() and ArrayOneItem on the statuses() result.
• Needs the STRYKER_DASHBOARD_API_KEY secret (repo or org) for the dashboard upload.

Review notes

• Infection is installed with composer require in the workflow, not from the phar: the Testo adapter bundled in the 0.35.6 phar writes a bootstrap calling the unscoped \Infection\StreamWrapper\IncludeInterceptor, so every mutant ends as an error.
• It is not in require-dev because Infection ≥ 0.33 needs PHP 8.3+, and the package supports 8.2.

Checklist

• Tested
  • Infection run locally on PHP 8.4 with Xdebug

[coderabbitai]

Important

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This repository does not receive automatic reviews because it has fewer than 10 stars.

⚙️ Run configuration

• Configuration used: defaults
• Review profile: CHILL
• Plan: Advanced
• Run ID: 4af74776-b2f2-434d-a97f-606f07d245fc

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