Add bulk item consumer creation GraphQL mutation

What does this MR do and why?

Adds a new AiCatalogItemConsumerBulkCreate GraphQL mutation that enables an AI catalog agent or flow across multiple projects at once asynchronously via Sidekiq.

This is the backend portion of Iteration 1 — enabling bulk selection of individual projects for agent/flow enablement.

The mutation is gated behind the ai_catalog_bulk_item_consumer_create feature flag.

What's remaining

• Catch and log errors when creating the consumers fails
• Add a limit to how many can be batch added at once
• Email a report of the results (We need to decide how to do this idempotently)
• Restrict all operations to a single organization
• Enable feature flag
• Remove feature flag

References

• Issue: #595242

How to set up and validate locally

1. Enable the feature flag:

   Feature.enable(:ai_catalog_bulk_item_consumer_create)

2. Run the GraphQL mutation:

   mutation {
     aiCatalogItemConsumerBulkCreate(input: {
       itemId: "gid://gitlab/Ai::Catalog::Item/<ITEM_ID>"
       projectIds: [
         "gid://gitlab/Project/<PROJECT_1_ID>",
         "gid://gitlab/Project/<PROJECT_2_ID>"
       ]
       triggerTypes: ["mention", "assign"]
     }) {
       errors
     }
   }

3. Verify the response has empty errors and item consumers were created:

   Ai::Catalog::ItemConsumer.where(ai_catalog_item_id: item.id).pluck(:project_id, :group_id)

4. Try again with a project or item you don't have access to, and verify that you receive an error message.