Can't paste image to /btw discussion

### Describe the bug

If i start prompt with /btw I can't paste images. But I can pages image to a prompt and then add /btw and that seems to work.

### Affected version

GitHub Copilot CLI 1.0.72-0

### Steps to reproduce the behavior

1. type `/btw`
2. try to paste image you have in clipboard
3. nothing

### Expected behavior

1. image gets pasted

### Additional context

_No response_

Start by reproducing the issue in GitHub Copilot CLI 1.0.72-0: begin a prompt with /btw and paste an image, then compare it with pasting an image before adding /btw. Trace the /btw input handling and clipboard-image path; done means an image pasted after /btw is accepted in the discussion.

Written by the indexing model from the issue text.