Add mutation to destroy AI catalog item

What does this MR do and why?

This MR introduces a GraphQL mutation to destroy AI catalog items. This allows users to delete catalog items they no longer need through the GraphQL API.

Implementation details

• Adds aiCatalogAgentDestroy mutation to the GraphQL API
• Accepts catalog item ID as input parameter
• Returns success message and any errors

SQL queries

DELETE FROM 
  "ai_catalog_items" 
WHERE 
  "ai_catalog_items"."id" = 10

DELETE FROM 
  "ai_catalog_item_versions" 
WHERE 
  "ai_catalog_item_versions"."id" = 9

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Switch to branch 553293-ai-catalog-agent-item-destroy
• Take any catalog item global id you want to delete.
• Execute below mutation from graphql console.

mutation {
  aiCatalogAgentDestroy(input: {
    id: "gid://gitlab/Ai::Catalog::Item/5"
  }) {
    success
    errors
  }
}

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 #553293 (closed)