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

[gustavlagneborg]

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

[Jaksenc]

Quick behavior check for this one: during streaming, if a composable PromptInput doesn’t include PromptInputSubmit, should Enter still submit the form, or should the keypress be ignored? PromptInputTextarea currently uses that button to decide whether Enter is blocked, and the existing prompt-input.test.tsx coverage is a clean place to pin down the expected behavior.