structured output support

[markacianfrani]

Steps to reproduce:

import { generateObject } from "ai";
// @ts-ignore
import { githubModels } from "@github/models";
import { z } from "zod";

const { object } = await generateObject({
  model: githubModels("openai/gpt-4o-mini"),
  schema: z.object({
    name: z.string(),
    age: z.number(),
  }),
  prompt: "Generate a person",
});

console.log(object);

The SDK has a nice abstraction around structured outputs but they don't currently work with compatible models.

Fix is very simple: just add supportsStructuredOutput to the provider. However, the hard part is that not all GitHub Models support it. and there's no metadata that I know of that can tell us which ones do and which ones don't

[markacianfrani]

On second thought, all of the current models actually do support structured output. I was just getting rate limited and some of the thinking models take awhile to return.

I think the simplest solution here is to just enable it for all models. The risk is that a new model gets added that doesn't support it (kinda unlikely?) and the user won't see the helpful AI SDK warning message:

AI SDK Warning System: To turn off warning logging, set the AI_SDK_LOG_WARNINGS global to false.
AI SDK Warning: The "responseFormat" setting is not supported by this model - JSON response format schema is only supported with structuredOutputs

I submitted a PR for this. I originally included the test script that would hit the GitHub model api and actually verify but it was pretty brittle, took too long, and used real requests. N