fix: restore test environment fallback mounts

ubaid ur rehmanrequested to merge
theredhacker0345/docs-gitlab-com:fix/test-environment-fallback-mounts into main

Summary

This restores the fallback-aware module.mounts inherited from the default Hugo configuration for the test environment. It also makes merge-request review-app builds select that environment. Together, these changes allow localized docs to resolve English-only content through the same fallback mounts used by the canonical configuration.

This intentionally includes only the configuration portion of the prior handover work. It does not restore the temporary shortcode workaround that was excluded from the final scope.

Closes #708.

Verification

  • Resolved the default and test Hugo configurations and verified that their module mounts are identical, including Japanese, French, and Korean English-fallback mounts.
  • Ran an isolated Hugo fixture that renders a Korean page referencing English-only GitLab documentation through the inherited fallback mount.
  • Ran the repository YAML linter for all YAML files and the edited CI configuration.
  • Ran git diff --check.

AI assistance

This contribution was prepared with AI assistance. I reviewed the patch, confirmed the issue and prior handover scope against the public project materials, and ran the validation listed above before submitting it.

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