Streaming terminal output not updating with async Terminal call

Testing https://github.com/microsoft/vscode/issues/327411

Ask the agent the following, and note that the output doesn't update

```
Run the terminal tool with these parameters
"parameters": {
"command": "Write-Host \"Starting long-running process...\"\nfor ($i = 1; $i -le 30; $i++) {\n Write-Host \"Step $i of 30 - $(Get-Date -Format 'HH:mm:ss')\"\n Start-Sleep -Seconds 1\n}\nWrite-Host \"Process complete!\"",
"description": "Run the long-running command again",
"initial_wait": 10,
"mode": "async"
}
```

Example screenshot:

Another related issue is that the little incomplete output that does show up actually disappears with "Terminal is no longer available" if you switch sessions: