Previous prompts lost after editing

### Describe the bug

Previous prompts are lost when editing them.

### Affected version

1.0.65

### Steps to reproduce the behavior

1. Go to a previous prompt (for example, using the up arrow)
2. Edit the prompt (clear it completely, or edit it partially)
3. Press enter

Original prompt is now lost. This is specially painful when the line was cleared completely.

### Expected behavior

Original prompt remains in history.

### Additional context

Ubuntu 24.04.4.
x86_64.
GNOME Terminal 3.52.0.
zsh 5.9.

No files or tests are named. First reproduce the issue in Copilot CLI under zsh by recalling a previous prompt, editing it, and pressing Enter; then trace the prompt-history handling. Done means edited prompts, including completely cleared ones, remain available in history.

由索引模型根据 Issue 内容生成。