Background tasks killed but agent still waiting

### Describe the bug

I used /tasks to cancel some long running sub agent tasks, but the main agent still seems to be waiting on background agents. I waited for over an hour to see if it would eventually stop, but it didn't, and pressing the Esc key didn't stop it either.

### Affected version

GitHub Copilot CLI 1.0.32

### Steps to reproduce the behavior

_No response_

### Expected behavior

_No response_

### Additional context

_No response_

Start by reproducing the issue in GitHub Copilot CLI 1.0.32: launch long-running background sub-agent tasks, cancel them with /tasks, and press Esc. Done means the main agent no longer waits after the background tasks are cancelled and interruption works as expected.

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