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)

No source file or test is named. Reproduce the issue by starting copilot, entering !, typing a filename, and pressing Tab in the reported Windows/WT/pwsh setup; then trace the CLI's Tab handling and shell-mode entry point. Done means Tab invokes shell completion in bash mode without changing tabs, while normal tab navigation remains unchanged.

Written by the indexing model from the issue text.