Fix PHPUnit matrix dependency resolution for PHP 8.1–8.3

[Copilot]

The PHPUnit workflow was mutating the CI Composer manifest for older PHP matrix entries while still installing from a lockfile pinned to PHPUnit 13. As a result, the PHP 8.1 job failed during dependency installation before PHPUnit could run.

• CI dependency resolution

  • Switch matrix entries that override phpunit/phpunit (8.1, 8.2, 8.3, and DB test jobs) from composer install to targeted composer update phpunit/phpunit --with-all-dependencies.
  • Keep the existing lockfile-based install path for matrix entries that do not rewrite the PHPUnit constraint (8.4, 8.5).

• Behavioral impact

  • Aligns Composer’s operation with the generated composer.ci-phpunit.json contents for each matrix leg.
  • Prevents lockfile/constraint mismatches when CI downgrades PHPUnit for older PHP versions.

• Workflow shape

  • Preserve the existing CI overlay approach and package set.
  • Limit the change to workflow logic; no production or test source changes.

if [[ -n "${PHPUNIT_VERSION:-}" ]]; then
  COMPOSER="${CI_COMPOSER_FILE}" composer update phpunit/phpunit \
    --with-all-dependencies --prefer-dist --no-progress --no-interaction \
    --ignore-platform-req=php --ignore-platform-req=ext-mongodb
else
  COMPOSER="${CI_COMPOSER_FILE}" composer install \
    --prefer-dist --no-progress --no-interaction \
    --ignore-platform-req=php --ignore-platform-req=ext-mongodb
fi