Drag and drop of files to attach no longer works in the Copilot app (regression, macOS)

### Describe the bug

Dragging a file from Finder onto the GitHub Copilot app to attach it to a prompt no longer works. When a file is dropped on the prompt composer, nothing is attached: no attachment chip appears and no file path is inserted.

This is a regression. Drag-and-drop file attach worked in a previous release (the feature was originally added in #363), and after updating to a newer build it is broken again, back to the same behavior as before the feature existed.

### Affected version

1.0.65

### Steps to reproduce the behavior

1. Open the GitHub Copilot app.
2. Focus the prompt composer in a session.
3. Drag a file (for example an image PNG, or a text/source file) from Finder and drop it onto the composer.
4. Observe that nothing is attached: no attachment chip is shown and the file path is not inserted into the prompt.

### Expected behavior

Dropping a file onto the composer should attach it to the prompt (show an attachment chip / insert the file path), the same way it worked in the release where this feature was functioning.

### Additional context

- Operating system: macOS
- CPU architecture: arm64 (Apple Silicon)
- Regression: drag-and-drop file attach worked in an earlier release and stopped working again after a later update.
- Related history: #363 (drag/drop + paste of images added), #2043 (related drag handling of a macOS screenshot temp path in a terminal).

まず macOS 1.0.65 で問題を再現し、次に現在の動作を #363 で導入されたドラッグアンドドロップ実装と比較します。macOS の関連するパス処理については #2043 を確認してください。画像またはテキスト/ソースファイルをプロンプトコンポーザーにドロップすると、期待される添付チップが作成されるか、そのパスが挿入されれば完了です。

索引モデルが issue の本文から書いたものです。