Add tests for Extension Gitlab pipeline build

[jordojordo]

Is your feature request related to a problem? Please describe.

Follow-on from #13032

We should add to the nightly extension workflow tests to ensure that the Gitlab reusable workflow (./shell/scripts/.gitlab/workflows/build-extension-catalog.gitlab-ci.yml) would pass in a Gitlab pipeline. This would also include the bundled workflow file that is shipped with the extension creator (./creators/extension/app/files/.gitlab-ci.yml).

Describe the solution you'd like

The simplest solution would be to replicate the GitLab CI logic as GitHub Actions steps with the same container image used.

Describe alternatives you've considered

• Run a GitLab Runner container within the GitHub Action
• Trigger a GitLab pipeline remotely from GitHub (would require a Rancher Gitlab org and repo)

Additional context

Since the build-extension-catalog.gitlab-ci.yml is fairly straightforward (it installs some packages, logs into a registry, etc.), the simplest way to test what GitLab does is to copy those steps into a GitHub workflow directly. Essentially, the GitHub Action would:

• Spin up a registry.suse.com/bci/bci-base runner
• Install the same system packages (podman, node.js, yq, etc.).
• Perform the same build steps (yarn publish-pkgs, podman push --dry-run, etc.).
• Check for collisions or version mismatches in the same way we do in check_version_collisions job.