@ 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_

在 GitHub Copilot CLI 1.0.27 中使用指向 c:\foo bar\file.md 的 @ 參照重現此問題，然後追蹤 @ 檔案自動完成和路徑解析的進入點。使用 Tab 和 Backspace 導覽越過空格，直到建議 file.md，以確認修正有效，同時保留文件中針對不含空格路徑的行為。

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