Copilot sessions: returning to a tunnel session sometimes leaves Send unresponsive

## What happens

I use VS Code desktop with **Copilot** selected as the session type, rather than **Local**.

After setting up a tunnel and coming back later, I sometimes cannot send messages. My text stays in the input box and nothing happens.

## Observed sequence

1. Set up a tunnel and use a Copilot session in VS Code desktop.
2. Leave and return later.
3. Type a message and try to send it.

This happens intermittently. I do not have a reliable reproduction or know whether the tunnel has disconnected when it happens.

## Expected behavior

Send the message, or show a connection error with a way to reconnect. Do not silently leave the message in the input box.

## Related reports

- #334573 concerns sending through a disconnected tunnel. I do not know whether this is the same problem.
- #334159 describes input clearing followed by a stuck turn. In my case, the text stays in the input box.

## Environment

VS Code desktop, Copilot session type, tunnel connection. Exact VS Code build and Copilot runtime version not yet collected. User-observed; not independently reproduced.