Backend: Add GraphQL mutation to destroy agent

About

Similar to the work in #551282 (closed) we want to add a GraphQL mutation to destroy an agent.

Proposal

🎗️ The mutation must be marked as experimental.

🎗️ Remember our frontend-backend collaboration guide and communicate schema ideas on this issue with the frontend counterpart of #550465 (closed).

What is destroyed?

This mutation will destroy the Item record itself and all of its versions (ItemVersion).

Schema

The arguments would be:

Argument name	Type	Required?
id	::Types::GlobalIDType[::Ai::Catalog::Item]	✅

The mutation would have one nullable field:

field name	Type	Nullable?
item	::Types::Ai::Catalog::ItemType	✅

Similar to other destroy mutations, the mutation will return a null item if the item is destroyed successfully, but the actual item when it is not destroyed successfully.

(The errors field) is defined for all mutations through BaseMutation).

Example mutation call:

 mutation {
   aiCatalogAgentDestroy(input: {
     id: "gid://gitlab/Ai::Catalog::Item/1"
   }) {
     errors
     item {
       id
     }
   }
 }

Must check item is an Agent

As agents are currently modelled in the same table as other AI items will be in, the Global ID type for an Agent will not guarantee that we are destroying an agent item as opposed to another other item.

The service must return an error response if the AI item is not an agent.

Authorization

The authorization for the mutation should be the same as in !196462 (merged) which is admin_ai_catalog_item at time of writing.