agent_ui: Fix show markdown list checked state (#43567)

Closes #37527

This PR adds support for showing the list state of a list item inside
the agent UI.

**Before**
<img width="643" height="505" alt="Screenshot 2025-11-26 at 16 21 31"
src="https://github.com/user-attachments/assets/30c78022-4096-4fe4-a6cc-db208d03900f"
/>

**After**
<img width="640" height="503" alt="Screenshot 2025-11-26 at 16 41 32"
src="https://github.com/user-attachments/assets/ece14172-79a5-4d5e-a577-4b87db04280f"
/>

Release Notes:
- Agent UI now show the checked state of a list item

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
2260b87ea8Remco Smits committed on 12/8/2025, 2:04:49 PM· committed by GitHubparente6214b2
2 files changedLine totals unavailable