agent_ui: Refine the subagent UI design (#46838)

This PR refines the design for the subagent card UI. Here's a quick
preview:

<img width="500" height="544" alt="Screenshot 2026-01-14 at 6  43@2x"
src="https://github.com/user-attachments/assets/a293b9ec-37cd-4098-b8ea-b321a239dea3"
/>

Release Notes:

- N/A
b5b13d1f4dDanilo Leal committed on 1/14/2026, 10:01:36 PM· committed by GitHubparent138c335
1 file changedLine totals unavailable