Add trigger agent selection dropdown

What does this MR do and why?

Add trigger agent selection dropdown

Allow users to select an Agent from the catalog to execute based on a trigger.

References

Screenshots or screen recordings

Screen Recording 2025-09-30 at 10.39.20.mov

How to set up and validate locally

1. Enable the feature flag: echo "Feature.enable(:ai_flow_triggers)" | rails c
2. Go to http://gdk.test:3000/gitlab-duo/test/-/automate/flow-triggers
3. Make sure you have service account users
4. Create a flow trigger
5. Notice the new radio option between config and Ai Catalog
6. Notice the new dropdown at the bottom
7. Click to select a flow
8. Save
9. Notice it works!
10. Load this workflow
11. Notice the value you picked is selected!

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.