Start using :read_ai_catalog_item ability

What does this MR do and why?

Starts using the :read_ai_catalog_item ability we introduced in !196886 (merged)

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the :global_ai_catalog feature flag
2. Make note of the item ids returned when you create them in the following steps
3. Create a public catalog item in any project (rails only, no API yet): FactoryBot.create(:ai_catalog_item, project_id: 1000000c, public: true)
4. Create a private catalog item in a project you are a developer+ of (rails only, no API yet): FactoryBot.create(:ai_catalog_item, project_id: 1000000c)
5. Create a private catalog item in a project you are a developer+ of (rails only, no API yet): FactoryBot.create(:ai_catalog_item, project_id: 1000000c)
6. Make a GraphQL query:

   {
     aiCatalogItems {
       nodes {
         id
       }
     }
   }

7. Ensure the public item and private one you are developer+ in the project appear, but the other one does not!
8. Query each item individually to ensure you can/can't access as expected:

    {
      aiCatalogItem(id: "gid://gitlab/Ai::Catalog::Item/3") {
        id
      }
    }

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #553928 (closed)