Enable Vue 3 migration rollout for namespace model selection page

What does this MR do and why?

Enables the Vue 3 migration rollout for the namespace-level (group) Model Selection entrypoint (ee/app/assets/javascripts/pages/groups/settings/gitlab_duo/model_selection, app ModelSelectionApp).

This uses the vue3_migration.yml mechanism (recommended Option 1) to opt the page's entrypoint into Vue 3 behind a feature flag.

What changed

• Added config/feature_flags/beta/vue3_migrate_namespace_model_selection.yml
• Added ee/app/assets/javascripts/pages/groups/settings/gitlab_duo/model_selection/vue3_migration.yml with status: rollout

How to set up and validate locally

1. Ensure you are on Vue 3 by following these instructions
2. Restart Vite: gdk restart vite
3. Enable the feature flag: Feature.enable(:vue3_migrate_group_model_selection)
4. Navigate to Group > Settings > GitLab Duo > Model selection
5. Open the browser console and confirm the message: [gitlab] [V] Using Vue.js 3 (with @vue/compat) for ModelSelectionApp
6. Confirm document.querySelectorAll('[data-gitlab-vue3-app]') returns the app
7. Verify the page renders and functions correctly

Verification

• Page built correctly with Vue3

  

• Page tested and working as expected.

References

• Migration issue: #607799
• Migration guide: https://docs.gitlab.com/development/fe_guide/vue3_migration/#option-1-recommended-migrate-your-page-entrypoint-using-a-feature-flag-and-vue3_migrationyml
• Example MR: !249743 (merged)