agent_ui: Move UI logic from `ThreadHistory` to separate component (#51301)

This is just a re-factor. We'll make use of this once we make thread
history non-global (storing one history per ACP connection).

Release Notes:

- N/A
dfc3a7c6e8Bennet Bo Fenner committed on 3/11/2026, 3:57:22 PM· committed by GitHubparent2b425aa
9 files changedLine totals unavailable