docs: fix nuxt instructions to use properly cased environment variable

[JessicaSachs]

Instructions are currently broken. Nuxt environment variables configured in runtimeConfig within nuxt.config.ts will retain case. We configured the process.env var to be called OPENAI_API_KEY, so the eventHandler needs to call it the same thing.

To reproduce, follow the docs:

// server/event.ts - some file with an event handler, from this tutorial
import { streamText } from 'ai';
import { createOpenAI } from '@ai-sdk/openai';

export default defineLazyEventHandler(async () => {
  debugger; // Inspect `useRuntimeConfig()`
  const apiKey = useRuntimeConfig().openaiApiKey;
  if (!apiKey) throw new Error('Missing OpenAI API key');
  const openai = createOpenAI({
    apiKey: apiKey,
  });

  return defineEventHandler(async (event: any) => {
    const { messages } = await readBody(event);

    const result = streamText({
      model: openai('gpt-4o'),
      messages,
    });

    return result.toDataStreamResponse();
  });
});

[lgrammel]

The docs are based on our (working) Nuxt example, see e.g. here: https://github.com/vercel/ai/blob/main/examples/nuxt-openai/server/api/chat.ts

I suspect that the difference stems from how you set up your nuxt.config.js - here is what we currently do: https://github.com/vercel/ai/blob/main/examples/nuxt-openai/nuxt.config.ts

That being said, I like having OPENAI_API_KEY better - but I want to keep all docs consistent, and also follow Nuxt best practices. What are the Nuxt best practices for naming such API keys in env variables?