fix(elements): block Enter submit while streaming in PromptInputTextarea

[mmaxence]

Fixes #439

While streaming with an onStop handler, PromptInputSubmit renders as type="button", so the Enter guard in PromptInputTextarea finds no button[type="submit"] and submitButton?.disabled evaluates to undefined: the form submits mid-stream. This applies the guard proposed in the issue: no submit button in the form blocks Enter, same as a disabled one.

This also answers the behaviour question raised in the issue thread: a composable PromptInput without PromptInputSubmit no longer submits on Enter. The form has no submit control to consult, and submitting with unknown status is the unsafe default. Both behaviours are pinned in prompt-input.test.tsx.

All 952 package tests pass.

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