Vue 3 migration: Kubernetes cluster pages

Part of &23285.

This issue tracks the Vue 3 migration of the Kubernetes cluster page entrypoints, using Option 1 (vue3_migration.yml plus a feature flag).

Reachability

Most of these pages sit behind the certificate_based_clusters ops flag. The flag defaults to off. Admin cluster pages return 404 in all cases. The GitLab agent UI at ~/clusters_list stays live and supported.

Pages	Reachable by default?	Evidence
`groups`/`projects` clusters `index` (agent tab)	Yes. The agent tab always renders.	`app/controllers/clusters/clusters_controller.rb:11` (`index` is excepted from `ensure_feature_enabled!`); `app/assets/javascripts/clusters_list/components/clusters_main_view.vue:65` (only the certificate tab is hidden)
`groups`/`projects` clusters `show`, `edit`, `connect`	No. These 404 unless `certificate_based_clusters` is on.	`app/controllers/clusters/clusters_controller.rb:11,141-143`
All `admin/clusters` pages, including `index`	No. These always 404 when the flag is off.	`app/controllers/admin/clusters_controller.rb:6` (a second `before_action` with no `except:`)
`destroy` action, all three clusterables	No. This action never renders HTML.	`app/controllers/clusters/clusters_controller.rb:70-77` (redirect only)

Entrypoints & apps

This migration covers 13 entrypoints. It excludes the 3 */clusters/destroy pages. A separate cleanup merge request removes those, because they can never execute.

Entrypoint group	Vue apps mounted	Page-specific components
`*/clusters/index` (admin, groups, projects — 3 entrypoints)	`~/clusters_list`, mounted with `initVueApp` (gets the Vue 3 marker)	16, mostly in `clusters_list/components`. All three pages share the same set.
`*/clusters/show` (admin, groups, projects — 3 entrypoints)	`~/clusters/clusters_bundle` (raw `new Vue`, no marker) and `~/clusters/forms/show` (`initVueApp`, marker); the projects page also loads `~/clusters/gke_cluster_namespace`	6
`*/clusters/edit` (admin, groups, projects — 3 entrypoints)	`~/clusters/clusters_bundle` (raw `new Vue`, no marker)	5
`*/clusters/connect` (admin, groups, projects — 3 entrypoints)	`~/clusters/new_cluster`, mounted with `initVueApp` (marker)	1
`groups/clusters/index.js` (1 entrypoint)	`~/clusters/forms/show/index`, mounted with `initVueApp` (marker)	1, in `clusters/forms`

Across all 16 original pages, the component union is about 22 page-specific components.

Risks

~/clusters/clusters_bundle.js:104 mounts with the raw new Vue({ el, ... }) constructor, not initVueApp. This root gets no Vue 3 marker. A silent fallback to Vue 2 on this app would be invisible.
The cluster feature specs never stub certificate_based_clusters. The flag is not in the ops-flags-disabled-by-default list at spec/spec_helper.rb:289-296. The suite only tests the flag-on path. Production defaults to flag-off. So existing specs do not prove users can reach these pages.
Most of the migrated pages get real traffic only through the agent tab on */clusters/index. The rest depend on the deprecated certificate flag, so manual QA with the flag on is the only way to check them before rollout.

Tasks / MRs

Cleanup MR: remove the three unreachable */clusters/destroy bundles (see the companion merge request).
MR 1: add the vue3_migrate_clusters feature flag definition and add a vue3_migration.yml (status: rollout) to all 13 surviving entrypoints.
Verify the three */clusters/index pages render the agent tab correctly under Vue 3.
Verify the deprecated show, edit and connect pages render correctly under Vue 3, with certificate_based_clusters turned on locally.
Convert or manually check the raw new Vue app in clusters_bundle.js:104, since it mounts without the Vue 3 marker.
Write a Sentry query to detect Vue 3 rendering errors on these pages.
Ramp the vue3_migrate_clusters flag, per the rollout plan below.
Cleanup MR: move all 13 vue3_migration.yml files to status: migrated.

Rollout

1. Enable vue3_migrate_clusters for gitlab-org.
2. Enable for 10% of actors. Soak.
3. Enable for 50% of actors. Soak.
4. Enable for 100% of actors. Soak.
5. Set default_enabled: true.
6. Open the cleanup MR to set status: migrated.

Notes

Restart the Vite dev server after you add a vue3_migration.yml. Otherwise the entry map stays stale. The app never mounts, and no console error appears.
Local GDK runs Vue 2. Stub the vue3_migrate_clusters flag off in feature specs to check rendering.
To see the deprecated pages at all locally, turn on the certificate_based_clusters ops flag.