AI Agent list view

What does this MR do and why?

This MR extends Graphql types for Ai::Agents (!141684 - merged) to add an empty state and list view to the AI Agents UI.

Related to #437299 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Empty State

List View

Database Review

Finder Query

SELECT
  "ai_agents".*
FROM
  "ai_agents"
WHERE
  "ai_agents"."project_id" = 19

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/25451/commands/80595

How to set up and validate locally

1. Ensure you have an EE license activated

2. Enable the feature flag

   Feature.enable(:agent_registry)

3. Navigate to <project>/-/ml/agents and ensure that the empty state is loading correctly

4. Click on 'Create agent'

5. Create an agent (name cannot have spaces, prompt can be anything)

6. Navigate to <project>/-/ml/agents and ensure the agent list shows up with the item that was just created