Draft: Add artifact registry hosted repository delete mutation

What does this MR do and why?

Adds the hosted repository delete mutation for Artifact Registry (monolith/S07 Step 4), on top of the S07 GraphQL foundation (!248967 (merged)).

Mutations::ArtifactRegistry::Repositories::Delete takes the repository name (identity only) and calls the S02 client delete_repository, which is idempotent: it succeeds even on a 404. It returns the standard payload errors. All experiment (19.3), behind the dark artifact_registry_ui flag.

A flag-off delete raises a top-level ResourceNotAvailable with no client call; an AR 403 renders a top-level ResourceNotAvailable; a delete 404 succeeds idempotently rather than surfacing an error.

The frontend typedefs (graphql/typedefs.graphql) drop the local @client declaration of Mutation.artifactRegistryRepositoryDelete, which the merged Vue write UI declared while there was no backend. Now that this MR adds the real server mutation, keeping the local declaration would collide during the Apollo schema build. The delete flow still runs on the local mock resolver through the remaining @client operation.

References

• Spec: docs/specs/monolith/S07-repository-crud.md (Approved), Mutations and Error cases
• Plan: docs/plans/monolith/2026-07-17-repository-crud.md (merged), Step 4
• Builds on the S07 foundation: !248967 (merged)
• Shared type and enums: !248690 (merged) (monolith/S04), merged

Screenshots or screen recordings

N/A. Schema only, behind a dark flag.

How to set up and validate locally

1. Point the client at a local Artifact Registry service and enable the flag:

   org = Organizations::Organization.first
   Gitlab.config.artifact_registry['api_url'] = 'http://127.0.0.1:8090'
   Feature.enable(:artifact_registry_ui, org)

2. Delete an existing repository and confirm empty errors:

   mutation = <<~GQL
     mutation($name: String!) {
       artifactRegistryRepositoryDelete(input: { name: $name }) { errors }
     }
   GQL
   user = org.organization_users.first.user
   pp GitlabSchema.execute(mutation,
     variables: { 'name' => 'my-repo' },
     context: { current_user: user, current_organization: org }).to_h

3. Confirm deleting an already-absent repository still succeeds with empty errors (idempotent); an AR 403 renders a top-level ResourceNotAvailable; and with the flag off the mutation raises a top-level ResourceNotAvailable with no client call.

Request spec:

bundle exec rspec ee/spec/requests/api/graphql/mutations/artifact_registry/repositories/delete_spec.rb

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist.