Right now there is some margin that uses `editor background` above the
output of the terminal that carries `terminal_background`. If the editor
uses a light background and the terminal is dark you'll end up with a
light area of margin above the dark terminal. The terminal background
also clips the rounded border of the box. This patch switches this to
match the terminal background and makes sure the background never clips
the rounded border.

Before:
<img width="720" height="212" alt="Screenshot 000044 in Zed@2x"
src="https://github.com/user-attachments/assets/333a0e17-2869-4cc9-b696-f24eec8630aa"
/>


After: 
<img width="646" height="228" alt="Screenshot 000050 in zed@2x"
src="https://github.com/user-attachments/assets/54293de1-bc64-450f-b354-e64ba0e3d5e9"
/>

Release Notes:

- Fixed terminal tool output in the Agent Panel to consistently use the
theme’s `terminal.background` color.

---------

Co-authored-by: dino <dinojoaocosta@gmail.com>