Agent gets stuck when canceling sub-agent tasks

### Describe the bug

Agent is unable to cancel tasks being run on subagents. It gets stuck in a canceling state and doesn't accepts any new prompts or slash (/ ) commands

### Affected version

GitHub Copilot CLI 1.0.27

### Steps to reproduce the behavior

1. Use /fleet to run a large task in parallel
2. Press Esc to cancel

### Expected behavior

Agent cancels all subagent tasks successfully

### Additional context

_No response_

Reproduce the issue in GitHub Copilot CLI 1.0.27 by using /fleet for a large parallel task, then pressing Esc to cancel. Trace the cancellation flow for the sub-agent tasks and verify that cancellation completes and the agent accepts new prompts and slash commands afterward.

Written by the indexing model from the issue text.