fix: #3625500 Fix PHPStan next-major warning in AWS Cloud config test on 9.x

The 9.x pipeline began failing PHPStan after PHPStan 2.2.15 reported a redundant null-coalescing fallback for the selected AWS region code. Initialize the variable before the selection loops and pass it directly to the cloud-context helper, preserving the previous empty-string fallback if no region is selected.

Drupal core #3067979 subsequently removed test files from Composer dist packages. Until the corresponding GitLab template fix is included in a tagged release, the next-major Composer job follows the upstream main branch, which contains the fix, so Drupal core's test classes are available to PHPStan and PHPUnit. This temporary override can be removed after the recommended tag includes the fix.

Issue: https://www.drupal.org/project/cloud/issues/3625500

Validation: The merged-results pipeline #975649 passed regular and next-major Composer, PHPStan, PHPUnit, PHPCS, Composer lint, ESLint, and Stylelint. The next-major Composer artifact contains web/core/tests.