Add aiCatalogAgentsCreate mutation

What does this MR do and why?

We need a way to create items in the AI catalog

This MR introduces the first of those mechanisms with a service and mutation to create AI catalog agents

References

Screenshots or screen recordings

How to set up and validate locally

1. Ensure :global_ai_catalog feature flag is enabled (and Duo Workflow is available- license configured etc)
2. Execute the GraphQL mutation along the lines:

    mutation m {
      aiCatalogAgentsCreate(input: {
        name: "Pirate"
        description: "A pirate doing a sum"
        projectId: "gid://gitlab/Project/1000000"
        systemPrompt: "You are a pirate!"
        userPrompt: "What is 2 + 2?"
      }) {
        errors
        item {
          id
        }
      }
    }

3. Check the item and version was correctly created in the rails console: Ai::Catalog::Item.last // Ai::Catalog::Item.last.versions.first

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 #551282 (closed)