Using tab during file autocomplete adds space

### Describe the bug

Using tab during file autocomplete via '@' adds the selected file/folder along with a space at the end. The space at the end causes to exit out of the autocomplete view, making the user to backspace to continue autocomplete. This makes tab and enter virtually the same, which is not ideal.

### Affected version

1.0.14

### Steps to reproduce the behavior

1. start copilot
2. enter @
3. select a folder
4. hit tab

### Expected behavior

tab should enter the selected folder, without the space at the end. should not exit out of autocomplete view

### Additional context

_No response_

Start by reproducing the issue in Copilot CLI using the listed steps: enter @, select a folder, and press Tab. Read the file-autocomplete handling and verify how Tab inserts the selected path. Done means Tab enters the selected folder without a trailing space and keeps autocomplete open, while Enter retains its separate behavior.

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