Support propertyOrdering for google models

[jeff-an]

Description

Property ordering is extremely important when generating JSON. Google's API requires you to set a propertyOrdering key when sending requests to its API in order to guarantee a specific ordering: https://ai.google.dev/gemini-api/docs/structured-output?lang=node#property-ordering

This is currently not exposed to the user in the Vercel SDK. I tried setting it through the middleware but unfortunately in the actual provider implementation it's being ripped out.

While making this actually part of the interface might be difficult since this is not a standard json schema property, a one-liner way to support this would simply be to pass the property through if it exists here: https://github.com/vercel/ai/blob/main/packages/google/src/convert-json-schema-to-openapi-schema.ts#L33

i.e. It will be the user's responsibility to modify the json schema to have the propertyOrdering property, vercel SDK will simply pass it through to Vertex if it exists.