Zod types via `infer` make TypeScript types a lot less readable

[felixweinberger]

There's a lot of indirection in the types via Infer<...> making types in IDEs significantly less readable and ergonomic than just standard TS Types.

Should likely explore ways to improve this if possible.

[reachaadrika]

hey @felixweinberger
I would like to work on this , can you assign this to me?

[KKonstantinov]

We could define the types separately.

Then we could use the satisfies keyword to make sure they match (although because of TS's structured typing system, we would have to be careful with that as satisfies wouldn't flag any missing fields in the schemas that are otherwise optional).

Not starting from schemas and then inferring, but rather starting from types and then defining schemas to match them, would allow us to make better use of type generics, so to have proper type checking on some types. (One example could be found at #859, where currently we only do a runtime check of structuredOutput, and we could really have it checked by the type system (e.g. if you defined outputSchema, then structuredOutput is required by the type system, and we would not rely on just a runtime check). However there is a multitude of opportunities like that, and the branch is just demonstrating one use case)

If the vitest migration PR (#1015) gets merged, we could further make use of their type testing feature (https://vitest.dev/guide/testing-types).

Happy to take this on as well, if it's a priority - but would like to align on the approach beforehand as it is a not insignificant piece, while having to be very careful for not breaking any compatibility as well.