fix(terminal): fill xterm-screen gap with terminal background

The xterm-screen height is row-aligned (rows × charHeight), leaving a
small gap at the bottom of the container. Adding the terminal background
to .terminal-instance-container ensures the gap blends seamlessly.

Author: abose

src/styles/Extn-Terminal.less

@@ -372,6 +372,7 @@
     bottom: 0;
     display: none;
     box-sizing: border-box;
+    background: var(--terminal-background);
 }
 
 .terminal-instance-container.active {