Schema output with createUnified

I was using the same code which is working fine with `createWorkersAI()` to generate schema output with CF hosted models, but when the createUnified() is used to generate a response with external model - The schema is not passed to the provider.

For example, I am using `google-ai-studio/gemini-2.5-flash-lite` here, and I see only `response_type: object` is sent and not the actual schema.

```
const aiGateway = createAiGateway({
accountId: env.CF_ACCOUNT_ID,
gateway: 'test',
});
const unified = createUnified({ apiKey: env.GEMINI_APIKEY });

const model = aiGateway(unified("google-ai-studio/gemini-2.5-flash-lite"));

const schema = Output.object({
schema: z.object({
priority: z.enum(["low", "medium", "high", "urgent"]).optional(),
replyMessage: z.string(),
}),
});

const result = await generateText({
model: model,
messages: messagesArray,
output: schema,
});

```

Schema output is supported in gemini-2.5-flash-lite -

https://ai.google.dev/gemini-api/docs/structured-output?example=recipe#model_support

Start by tracing the createUnified() model path used through createAiGateway(), then compare it with createWorkersAI() while handling generateText() output via Output.object. Reproduce the request with google-ai-studio/gemini-2.5-flash-lite and verify that the provider receives the actual schema rather than only response_type: object.

Written by the indexing model from the issue text.