Copilot sessions: pressing Enter takes several seconds to send a message

## What happens

In VS Code desktop, with **Copilot** selected as the session type rather than **Local**, pressing Enter takes a few seconds to send my message.

This report is about the delay in sending the message, not how long the model takes to answer.

## Observed sequence

1. Open a Copilot session in VS Code desktop.
2. Type a message.
3. Press Enter.
4. Wait a few seconds before the message sends.

I have not measured the delay or established a reliable reproduction.

## Expected behavior

Register the submission immediately. If sending needs to wait, show that the message is pending.

## Related report

#299606 describes a similar delay in Copilot Chat and is closed. I have not confirmed whether my Copilot session uses the same affected path.

## Environment

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