Support OpenAI's structured outputs mode

• currently only JSON mode is supported
• there are quite a few schema constraints when using structured outputs, and many of the AI functions in the stdlib use features like .optional(), .default(), etc which are not supported
• zod-to-json-schema both openai's vendored version and the official package with openai mode don't support structured outputs
• see the ai sdk's notes on structured outputs. they explicitly make them opt-in and warn about schema compatibility
• this will involve changes to the underlying zod-to-json-schema IF the user is using structured outputs, so we'll need to expose a different version of the AI functions in this case

see #702 for a repro