fix: block Enter submission when submit button is not in DOM

[HankGrimm]

What

Changes the Enter-key guard in PromptInputTextarea from submitButton?.disabled to !submitButton || submitButton.disabled.

Why

When PromptInputSubmit renders as a stop button during streaming (type="button"), form.querySelector('button[type="submit"]') returns null. The old guard evaluates null?.disabled to undefined, which is falsy, so the guard never fires — Enter submits a new message mid-stream.

Note on behavior change

This also means: if a composed PromptInput doesn't render PromptInputSubmit at all, Enter will now always be blocked (no submit button is ever found in the DOM). Flagging this per the discussion on the issue — happy to adjust if a different default is preferred for that composition case.

How verified

Traced the DOM query and type logic; no test suite found for this component (no prompt-input.test.tsx in the repo) — verification is by code reading, not by running tests.

Fixes #439

[vercel]

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