Restart to Update prompts once per window for active agent sessions

### Description

Follow-up to #332140. /cc @dmitrivMS

When **Restart to Update** is used with several VS Code windows that have active local agent sessions, every affected window blocks shutdown with an "A session is in progress" confirmation. The user has to find and confirm each window before the update can proceed.

Because **Restart to Update** is an explicit request to close and restore all windows, could this shutdown path skip the per-window active-session confirmation? If protection is still needed, a single app-level confirmation would be preferable to one prompt per window.

The confirmation should remain for ordinary quit, last-window close, workspace change, and reload operations.

### Steps to reproduce

1. Open several VS Code Insiders windows.
2. Start a local agent session in more than one window.
3. Wait until an update is ready.
4. Select **Restart to Update**.

### Actual behavior

Each window with an active session prompts separately before it can close, so the user must walk through all windows and confirm them one by one.

### Expected behavior

An explicit **Restart to Update** proceeds without per-window active-session confirmations, while normal close/quit protections remain unchanged.

### Context

Observed on Windows with VS Code Insiders on 2026-09-11. This request is specifically about the confirmation UX; the separate updater relaunch/process-ordering problem is being addressed independently.