Reconsider custom Agent selector gitlab-ui component

The following discussion from !215901 (merged) should be addressed:

• @tbulva started a discussion: (+2 comments)

  Question (non-blocking): To my understanding. What is this solving? I'd say using the select is the “recommended” solution.

  this is the core of the fix here: instead of listerning to select event on gl-collapsible-listbox that is not suitable for our need (maybe the whole component isn't suitable and we should find a more appropriate one), we rely on the simple click on the individual items in the list

  This statement is not explaining much.

Thanks for providing this fix, Denys! I agree it does the job, though it's a bit hacky. Given the headaches gl-collapsible-listbox has caused us, it may not be the most suitable gitlab-ui component for this problem.

Since responsibility for this component likely sits between group::duoChat and group::workflowCatalog, I'll create a follow-up issue from this thread and assign it to myself.

I'm not entirely sure if this needs fixing with duo-ui-next coming soon, but it's definitely something to revisit in the new year. FYI: @samdbeckham