Fix build-test Actions failure by pinning Composer to PHP 8.4

[Copilot]

build-test failed during dependency install because the lockfile now resolves doctrine/instantiator to a version requiring php ^8.4, while the workflow’s Composer action was executing on PHP 8.2. This PR aligns the workflow runtime with the lockfile requirements.

• Workflow change

  • Updated .github/workflows/phpunit.yml to set php_version: "8.4" on php-actions/composer@v6 in the build-test job.

• Why this resolves the failure

  • Composer dependency resolution now runs under a PHP version compatible with the locked dev dependency graph used by PHPUnit.

- name: Composer
  uses: php-actions/composer@v6
  with:
    php_version: "8.4"