Queued messages cannot be cleared without canceling running job

### Describe the bug

**Fix request:** Clear queued message without canceling running job

When the agent is actively working on a task, I can type a follow-up message that gets queued. However, there's no way to clear or edit
that queued message without canceling the running job. Both Ctrl+C and Esc cancel the current operation. Keybindings like Ctrl+U don't
appear to work on queued input.

**Expected:** A keybinding (e.g. Ctrl+U or a dedicated shortcut) that clears only the queued message text while leaving the running job intact.

**Current workaround:** None. You must cancel the job and re-prompt it.

### Affected version

GitHub Copilot CLI 1.0.5

### Steps to reproduce the behavior

Queue a respons with Ctrl-Q. Then try to clear it without stopping/interrupting a previously in-progress job.

### Expected behavior

A keybinding (e.g. Ctrl+U or a dedicated shortcut) that clears only the queued message text while leaving the running job intact.

### Additional context

_No response_