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)

未指定原始檔案或測試。透過啟動 copilot、輸入 !、鍵入檔案名稱，並在回報的 Windows/WT/pwsh 設定中按下 Tab 來重現問題；然後追蹤 CLI 對 Tab 的處理以及進入 shell-mode 的入口點。完成標準是：Tab 在 bash 模式中呼叫 shell 補全而不變更定位點，同時正常的 Tab 導覽維持不變。

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