Add aiCatalogAgentUpdate mutation

What does this MR do and why?

This adds a aiCatalogAgentUpdate mutation to update an agent version.

In this iteration, we just update the single version that was created in the create mutation. In a future iteration we will add extra logic around creating releases.

• #553290 (closed)

How to set up and validate locally

Enable the global_ai_catalog flag:

Feature.enable(:global_ai_catalog)

Create a new agent if one doesn't already exist:

Visit http://gdk.test:3000/-/graphql-explorer and use the following mutation:

 mutation {
   aiCatalogAgentCreate(input: {
     name: "name"
     description: "A pirate doing a sum"
     projectId: "gid://gitlab/Project/1000000"
     public: true,
     systemPrompt: "123123"
     userPrompt: "asdasd"
   }) {
     errors
     item {
       id
     }
   }
 }

Now, update the agent through the new migration added in this MR. Replace <GLOBAL_ID>, with the Global ID of an agent.

 mutation {
   aiCatalogAgentUpdate(input: 
    { 
      id: "<GLOBAL_ID>", 
      name: "New name", 
      description: "New description", 
      public: true,
      userPrompt: "New user prompt", 
      systemPrompt: "New system prompt"
    }
  ) {
    errors
    item {
      name
      description
      public
      versions {
        nodes {
          ...on AiCatalogAgentVersion {
            userPrompt
            systemPrompt
          }
        }
      }
    }
  }
 }

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Related to #553290 (closed)