Add AI catalog search (and expose via GraphQL)

What does this MR do and why?

Related to #555554

We need a way to search for catalog items. This MR adds the search and exposes it via GraphQL

References

Screenshots or screen recordings

How to set up and validate locally

1. Enable :global_ai_catalog feature flag
2. Create some agents (and take note of the id):

    mutation m {
      aiCatalogAgentCreate(input: {
        name: "Agent 1"
        description: "The best agent ever"
        projectId: "gid://gitlab/Project/1000000"
        systemPrompt: "Hello"
        userPrompt: "World"
        public: true
      }) {
        errors
        item {
          id
        }
      }
    }

3. Search in GraphQL explorer:

    {
      aiCatalogItems(
        search: "best"
      ) {
        count
        pageInfo {
          endCursor
          hasNextPage
        }
        nodes {
          id     
          itemType
          name
          description
        }
      }
    }

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