Group header and session list item overlaps buggily

### Description

1. Hovering on session list item of a session with sub-chats shows its chevron for collapsing/expanding the sub-chats above the group header
2. Hovering on group header renders it transparently causing the session list item below it to shine through

### Issue Details

Item|Value
---|---
Issue Category|Bug
Target|Agents Window
VS Code Version|Code - Insiders 1.137.0-insider (023b8803f23ea6451959d09a05c4e2ff90fedc34, 2026-09-08T00:36:40Z)
OS Version|Darwin arm64 25.6.0

System Info

Item|Value
---|---
VS Code Version|Code - Insiders 1.137.0-insider (023b8803f23ea6451959d09a05c4e2ff90fedc34, 2026-09-08T00:36:40Z)
OS Version|Darwin arm64 25.6.0
CPUs|Apple M5 Max (18 x 2400)
GPU Status|2d_canvas: enabled
GPU0: VENDOR= 0x106b [Google Inc. (Apple)], DEVICE=0x0000 [ANGLE (Apple, ANGLE Metal Renderer: Apple M5 Max, Version 26.6.2 (Build 25G83))], DRIVER_VENDOR=Apple, DRIVER_VERSION=26.6.2 *ACTIVE*
Machine model name: Mac
Machine model version: 17.6
direct_rendering_display_compositor: disabled_off_ok
gpu_compositing: enabled
multiple_raster_threads: enabled_on
opengl: enabled_on
rasterization: enabled
raw_draw: disabled_off_ok
skia_graphite: enabled_on
trees_in_viz: disabled_off
video_decode: enabled
video_encode: enabled
webgl: enabled
webgpu: enabled
webnn: disabled_off
Load (avg)|28, 32, 25
Memory (System)|128.00GB (33.58GB free)
Process Argv|--crash-reporter-id f3441869-abe8-416f-bbd5-01c71ecec52f
Screen Reader|no
VM|0%
User Agent|Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Code-Insiders/1.137.0-insider Chrome/148.0.7778.280 Electron/42.10.0 Safari/537.36
Installation pure|true

### Attachments