Cancellation: Receiver Hangs Indefinitely after entering code, when Sender has already cancelled

[donpui]

Describe the bug

When the sender cancels before the receiver accepts the transfer, the receiver hangs when it should terminate as well.

To reproduce

Steps to reproduce the behavior:

1. Sender: Starts send <file_path>
2. Receiver: Opens a link or enter
3. Sender: Cancel send
4. Receiver: Press Download
5. Receiver: Download button is deactivated and hanging

Expected behavior

Either the accept/reject flow is interrupted because Sender has cancelled or the transfer immediately fails after Receiver accepts.

** Note **
Initial issue is with backend (wormhole-william) part, where probably it should be fixed first.

Original issue: #OP858