Followup - UX revisions for pinned context

Tracking some of the reviewer feedback, question and assumptions made on feat(GlDuoChat): display attached context in ch... (!4540 - merged) to be sure they doesn't get lost.

1. Using chevron icons for collapsible context items section link
2. Should we prevent collapsing context items in the user message like the original implementation? How should we decide when to collapse? link
3. Related, should the context even appear on the assistant message? It's just repeated info since it's already displayed on the user message link, same feedback again
4. Related agaim do we really need to show the context in the prompt after it has been sent out? What's the point? Isn't the information in the answer enough? link
5. Should we try to extract the GitLab markdown issue/MR popup and use that when hovering an issue/MR context item? link
6. From the perspective of a user, how is the added context, if used by AI in the answer, different from "Source"? Should we treat context/sources as related concepts, we display them quite separately in this initial implementation link