[ResponsesAPI] return openai-defined types

[Wauplin]

PR on top of #1576.

I'm starting to think that it makes sense to define Zod schemas for inputs as we need to validate user's inputs. But that for outputs we "only" need static type checking and therefore we could reuse the types defined in https://github.com/openai/openai-node.

Benefits: no need to redefine stuff manually. It's easy to make mistakes (a parameter that shouldn't be nullable, that could be an array, etc.) when translating from the specs to our codebase. If static typing don't complain then we can assume "it's good".
Also less code to maintain.

Drawback: less flexibility. We don't own the stack and things might get updated in the wild. It's less a problem in this context as it's a server and not a client (and therefore we manage the dependency updates).

Overall I do think it's better to import from openai. Since we won't implement everything at first, it's fine to use Omit<..., "key-that-we-dont-implement"> which explicitly removes a feature (better than implicit non-definition)

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EDIT: and it's fine to use them for now and if it's ever blocking in the future, then we redefine stuff ourselves.

[Wauplin]

(merging as I think it'll be beneficial but open to discussion if anyone feel strongly)