fix: raise active pane border z-index above xterm-screen to prevent bottom clipping

[j4rviscmd]

Summary

The active pane border (::after pseudo-element) had z-index:6, which was below
.xterm-screen (z-index:31). This caused the terminal canvas to cover the bottom
border of the active pane in terminal-in-editor groups, while normal text editor
panes were unaffected.

Root Cause

.xterm-screen (from xterm.js) uses z-index:31 with position:relative. The active
pane border ::after was at z-index:6. Since both participate in the same stacking
context, the xterm canvas rendered on top of the border. The xterm element extends to
the very bottom of the editor group container, covering the bottom edge of the border.

The top border remained visible because xterm starts below the title bar area (xterm
top=64px vs group top=32px).

Fix

Raised z-index from 6 to 35:

• Above all xterm layers (max z-index: 34 - terminal-drop-overlay)
• Below hover/suggest widgets (z-index: 40+)
• pointer-events: none ensures no interaction blocking

Files Changed

• src/vs/workbench/browser/parts/editor/media/editorgroupview.css — editor group border
• src/vs/workbench/browser/parts/media/paneCompositePart.css — sidebar/panel/auxiliarybar border

Test Plan

• Terminal in Editor pane shows active border on all 4 sides
• Normal text editor pane border unchanged
• Hover popups render above the active border (not behind)