Agent Host sub-agent completion triggers premature native notification on Windows

## Description

When a sub-agent finishes in an Agent Host chat on Windows, VS Code emits a native completion notification. A single parent turn can invoke multiple sub-agents, so this can produce multiple notifications and can make it appear that the overall task has finished while the parent agent is still processing the sub-agent results.

This has been observed on Windows. It is not yet known whether macOS or Linux are also affected.

## Steps to reproduce

1. On Windows, start an Agent Host chat.
2. Ask the agent to delegate work to one or more sub-agents.
3. Let a sub-agent finish while the parent agent is still active.
4. Observe a native Windows notification when the sub-agent completes.

## Expected behavior

Sub-agent completion should not be treated as completion of the user-facing parent turn or session. A native completion notification should fire only when the parent turn/session finishes or needs user attention.

## Actual behavior

Each finishing sub-agent can produce a native completion notification even though the parent agent has not finished.

## Platform scope

- Reproduced: Windows
- Not yet verified: macOS, Linux