test(layout): add comprehensive tests for CortexBottomPanelContainer

Add 33 tests covering:
- Collapsed/expanded state rendering
- Tab rendering (terminal, output, problems)
- Tab switching via click handlers
- Active tab highlighting (border color, text color)
- Close button behavior
- Panel height application
- Resize handle (mousedown/mousemove/mouseup sequence)
- Height constraints (min/max clamping)
- Output panel onClose integration
- Resize handle hover effects

Author: echobt