Unarchive descendants when toggling group's archive state

What does this MR do and why?

When toggling the archive state of a group, automatically unarchive its descendant groups and projects.

This is done sync as we don't expect a group to have a large number of archived descendants. Otherwise, we can consider moving the operation into a worker later on.

Query Plans

NamespaceSettings Update: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/46270/commands/141225

Project Update: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/46270/commands/141234

References

Related to #578499

Screenshots or screen recordings

Before	After
Screen_Recording_2025-12-09_at_12.35.41_PM	Screen_Recording_2025-12-09_at_12.34.53_PM

How to set up and validate locally

1. Enable the cascade_unarchive_group on http://localhost:3000/rails/features/
2. Create the following group/project structure: Parent > Subgroup > Project
3. Go to http://localhost:3000/parent/subgroup/project/edit#js-project-advanced-settings and archive the project.
4. Go to http://localhost:3000/groups/parent/subgroup/-/edit#js-advanced-settings to archive the subgroup.
5. Go to http://localhost:3000/groups/parent/-/edit#js-advanced-settings to archive the top-level group.
6. Go to http://localhost:3000/groups/parent/-/edit#js-advanced-settings to unarchive the top-level group.
7. Verify that all groups and projects in the hierarchy are now in an active state.

MR acceptance checklist

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