Enter key submits form during streaming because querySelector('button[type="submit"]') returns null

Bug

When PromptInputSubmit is in the generating/streaming state it renders with
type="button" instead of type="submit" to act as a stop button. This means
form.querySelector('button[type="submit"]') in PromptInputTextarea returns
null.

The Enter key guard checks submitButton?.disabled, which evaluates to
undefined when submitButton is null — so the guard never fires and
form.requestSubmit() is called anyway, allowing users to submit a new message
while the model is still streaming.

Fix

// before
if (submitButton?.disabled) {

// after
if (!submitButton || submitButton.disabled) {
No submit button in the DOM → we're in stop/generating mode → block submission.

Steps to reproduce

1. Send a message and wait for the model to start streaming
2. Type anything in the textarea and press Enter
3. The form submits mid-stream

Start in PromptInputTextarea and trace the Enter-key guard that queries the form for button[type="submit"], alongside PromptInputSubmit's generating/streaming state. Verify the behavior when no submit button is present, then reproduce the streaming scenario and confirm Enter no longer calls form.requestSubmit() mid-stream.