Add append helper to useAssistant.

[lgrammel]

No description provided.

[AlexIsMaking]

@lgrammel thanks again but I'm afraid I've run into another issue - not 100% sure it's not me this time.

If I use:

append({
  role: "user",
  content: "hello world",
});

then the message field in input here, from my route, is an empty string

export async function POST(req) {
  const input = await req.json();
  console.log("input:", input);

like

input: {
  threadId: 'thread_abc123',
  message: ''
}

[lgrammel]

@AlexIsMaking #1425