Chat's "Open Changes in Diff Editor" doesn't work when the active editor is a webview

1. Have Chat make some changes
2. Open a webview, such as a PR description from GHPR
3. Use the "Open Changes in Diff Editor" action on one of the files in Chat
4. Nothing happens

<img width="618" height="273" alt="Image" src="https://github.com/user-attachments/assets/9a026b3b-2f9b-4071-8964-be2f316b2cf8" />