fix: #3625255 Fix Hetzner Cloud SDK dependency on 9.x

Problem

Cloud 9.x pins lkdevelopment/hetzner-cloud-php-sdk to an upstream development ref that is no longer available. Composer dependency resolution fails, so CI cannot reach the test stages.

Change

• Use the published ^3.1.2 SDK release in composer.json.
• Update existing functional tests to use the core Stark theme instead of Claro, which Drupal 12 removed, and assert the expected error text without depending on deprecated XPath or theme-specific markup. These pre-existing failures were exposed once Composer could install dependencies.

Tracked in #3625255.

Validation

• composer validate --no-check-publish --no-plugins
• PHP syntax check for every changed test file
• CI on this MR, including Composer resolution on current and next-major Drupal core