Fix AiTextToImageOutput type from UInt8Array -> ReadableStream

[dochne]

As noted in https://github.com/cloudflare/workerd/issues/2470 - the response type from env.AI.run with (in my case) @cf/bytedance/stable-diffusion-xl-lightning claims it should return a UInt8Array, but actually returns a ReadableStream.

This is fine when feeding it into a new Response() and returning, but is problematic when trying to use the data in other ways.

You can replicate this with the example model usage here

[ai]
binding = "AI"

export interface Env {
  AI: Ai;
}

export default {
  async fetch(request, env, ctx): Promise<Response> {
  const inputs = {prompt: "cyberpunk cat"};

  const response = await env.AI.run(
    "@cf/bytedance/stable-diffusion-xl-lightning",
    inputs
  );

  // Expected: Uint8Array. Actual: ReadableStream
  console.log(response);
  return new Response("Don't error");
}

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[dochne]

I have read the CLA Document and I hereby sign the CLA