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).

Bắt đầu bằng cách tái hiện lỗi trên macOS 1.0.65, sau đó so sánh hành vi hiện tại với phần triển khai kéo và thả được giới thiệu trong #363. Xem #2043 để biết cách xử lý đường dẫn liên quan trên macOS. Được xem là hoàn tất khi thả một hình ảnh hoặc tệp văn bản/mã nguồn vào trình soạn prompt tạo ra chip tệp đính kèm như mong đợi hoặc chèn đường dẫn của tệp.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.