Update terminal heading layout and hide the page behind certificate_based_clusters ff

Anna Vovchenkorequested to merge
255916-terminal-heading-layout-is-incorrect into master

What does this MR do and why?

This MR fixes the layout issue for the environment terminal. The feature is deprecated and should only be available behind the feature flag certificate_based_clusters. This MR also adds a check for the FF before rendering the page.

References

Screenshots or screen recordings

Before After
Screenshot_2025-09-04_at_22.04.46 Screenshot_2025-09-04_at_22.12.07
Screenshot_2025-09-04_at_22.15.10 Screenshot_2025-09-04_at_22.12.56

How to set up and validate locally

  1. Enable certificate_based_clusters feature flag
  2. Visit Operate -> Environments and create an environment
  3. Visit the environment page and add /terminal at the end of the URL (example: https://gdk.test:3443/gitlab-org/pipeline-variables/-/environments/28/terminal)
  4. Verify the layout looks fine.
  5. Disable the certificate_based_clusters feature flag
  6. Verify the page returns 404.

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Related to #255916 (closed)

Edited by Anna Vovchenko

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