sendMessageStreaming method
Returns a JS AsyncIterable over {content: [{type, text}, ...]} chunks.
Typed JSObject because dart:js_interop has no AsyncIterable type —
we obtain the iterator via [Symbol.asyncIterator]() on the Dart side.
Per the upstream TypeScript declarations message is
MessageLike | MessageLike[] (= string | Message | Array<...>).
We type as JSAny so callers can pass either text.toJS for the
text-only path or a jsify({role: 'user', content: [...] }) object
for multimodal content.
Implementation
external JSObject sendMessageStreaming(JSAny message);