Function start

tests/reasoning.test.tsx:66–70

(controller)

      '{"kind":"reasoning","delta":"a"}\n{"kind":"content","delta":"html"}\n';
    const body = new ReadableStream({
      start(controller) {
        controller.enqueue(new TextEncoder().encode(payload.slice(0, 24)));
        controller.enqueue(new TextEncoder().encode(payload.slice(24)));
        controller.close();
      },
    });

    const parts: string[] = [];