@ file autocomplete breaks on paths containing spaces

### Describe the bug

Autocomplete for @ file references doesn't seem to work for navigating into a folder with a space in the path.

### Affected version

GitHub Copilot CLI 1.0.27

### Steps to reproduce the behavior

1. Initial condition: A file exists at `c:\foo bar\file.md`
2. Type `@c:\foo` in the prompt
3. Autocomplete suggests `@c:\foo bar\` (a folder with a space in its name)
4. Press Tab to complete the path segment
5. Press Backspace to remove [the space](https://github.com/github/copilot-cli/issues/2429) and continue navigating

### Expected behavior

Autocomplete continues showing the folder's contents, allowing further navigation into the folder (e.g., to select `file.md`).

### Actual behavior

The autocomplete menu disappears and path resolution stops. The space in "foo bar" terminates autocomplete for the @ file reference. There appears to be no way to use autocomplete to navigate into files inside directories whose names contain spaces. (Quoting the path from the beginning doesn't trigger autocomplete at all.)

### Additional context

_No response_

Reproduce the issue in GitHub Copilot CLI 1.0.27 using an @ reference to c:\foo bar\file.md, then trace the @ file autocomplete and path-resolution entry points. Confirm the fix by navigating past the space with Tab and Backspace until file.md is suggested, while preserving the documented behavior for paths without spaces.

Written by the indexing model from the issue text.