Expose AiCatalogItem soft deleted fields

What does this MR do and why?

Expose AiCatalogItem soft-deleted fields in GraphQL for AI Catalog items.

The fields are Experimental are so have not been released, and so no changelog is required.

• https://gitlab.com/gitlab-org/gitlab/-/issues/575127

References

How to set up and validate locally

Enable the global_ai_catalog flag

Patch your local to allow any project to have AI Catalog items

--- a/ee/app/models/ee/project.rb
+++ b/ee/app/models/ee/project.rb
@@ -624,6 +624,7 @@ def suggested_reviewers_available?
      end

      def ai_catalog_available?
+        return true
      ::Gitlab::Llm::StageCheck.available?(self, :ai_catalog)
      end
      strong_memoize_attr :ai_catalog_available?

On your rails console, create 2 AI Catalog items; one soft-deleted and one not:

project = Project.last
FactoryBot.create(:ai_catalog_agent, project: project)
FactoryBot.create(:ai_catalog_agent, project: project, deleted_at: Time.now)

In http://gdk.test:3000/-/graphql-explorer, query for the item, replacing the <ID> with the ID of each of the items you created:

{
  aiCatalogItem(id: "gid://gitlab/Ai::Catalog::Item/<ID>", showSoftDeleted: true) {
    id
    softDeletedAt
    softDeleted
  }
}

For the soft-deleted item, you should see a response similar to:

{
  "data": {
    "aiCatalogItem": {
      "id": "gid://gitlab/Ai::Catalog::Item/203",
      "softDeletedAt": "2025-10-20T02:20:03Z",
      "softDeleted": true
    }
  },
  "correlationId": "01K7ZPXB276QFGPMFAHRKGBZHV"
}

For the non soft-deleted item, you should see a response similar to:

{
  "data": {
    "aiCatalogItem": {
      "id": "gid://gitlab/Ai::Catalog::Item/202",
      "softDeletedAt": null,
      "softDeleted": false
    }
  },
  "correlationId": "01K7ZPZTKR04ABKTG24A5QMMMD"
}

MR acceptance checklist

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Related to #575127