Support json-render in Think

[JSON-render](https://json-render.dev/) allows for generative UI and I _think_ it would be a good fit for Think. I managed to get it working by overriding `_transformInferenceResult` but it's a little ugly and I'm nervous about depending on a method that seems to be internal and only used for testing. Also the types don't seem right, but it does work:

```typescript
protected _transformInferenceResult(
result: StreamableResult,
): StreamableResult {
return {
toUIMessageStream(options) {
const stream = createUIMessageStream({
execute: async ({ writer }) => {
writer.merge(pipeJsonRender(result.toUIMessageStream() as any));
},
});
return stream as unknown as AsyncIterable;
},
};
}
```

It would be great if there was better support for this, or to understand whether there's an alternative approach that might fit the Think philosophy better?