Mark as Unread does not mark the currently focused session as unread

**Steps to reproduce**:
* Open the Agents window and focus a session.
* While that session remains focused, use Mark as Unread on it.
* Observe the session's unread state.

**Expected**: The focused session is marked as unread and displays its unread indicator.

**Actual**: Nothing changes; the session remains read. This appears to reproduce only when the session being marked as unread is currently focused.

Additional context: See the attached screen recording.

https://github.com/user-attachments/assets/2ef1bf40-1ff1-44f3-9d39-a702009599ac

```
Version: 1.139.0-insider
Commit: 046944034292b5479b4e9a50ad1a508033ffb64f
Date: 2026-09-17T03:41:14Z
Electron: 43.6.0
ElectronBuildId: 15274685
Chromium: 150.0.7871.250
Node.js: 24.20.0
V8: 15.0.245.31-electron.0
@github/copilot: 1.0.84-5
@github/copilot-sdk: 1.0.13
OS: Darwin arm64 25.6.0
```