When in bash mode (`!`) tab should autocomplete, not switch to next tab

### Describe the bug

When in bash mode (wrote `!`) tapping tab should autocomplete shell according to my shell rules, *not* switch to next tab. This is especially horrendous for trying to add filename as param to a shell command.

### Affected version

Copilot v1.0.70-0

### Steps to reproduce the behavior

1. `copilot`
2. `!`

### Expected behavior

File gets autocompleted (or any other shell command does).

### Additional context

- Windows / WT / pwsh.exe (but doubt that has any impact)

未指定源文件或测试。通过启动 copilot、输入 !、键入文件名，并在报告的 Windows/WT/pwsh 设置中按下 Tab 来复现问题；然后跟踪 CLI 对 Tab 的处理以及进入 shell-mode 的入口点。完成标准是：Tab 在 bash 模式下调用 shell 补全而不改变制表符，同时正常的 Tab 导航保持不变。

由索引模型根据 Issue 内容生成。