fix: optionality handling in zodv4 Properties type

[Dqu1J]

Issue

With the zodv4 schema the generated schemas fail to typecheck with their optional field types degrading to unknown. The issue is in the Properties type:

type Properties<T> = {
  [K in keyof T]: z.ZodType<T[K], T[K] | undefined>
}

In Zod v4, z.object determines whether a key is optional by checking the field schema's internal optout flag.
Currently the Properties type defines every field as a plain ZodType, which doesn't have that flag set, so optionality can't be inferred. That causes optional fields to degrade to unknown.

Fix

The Properties type has been changed as follows:

type Properties<T> = Required<{
  [K in keyof T]: undefined extends T[K]
    ? z.ZodType<T[K]> & { _zod: { optout: "optional" } }
    : z.ZodType<T[K]>
}>;

Optional fields (those that can be undefined) get their ZodType intersected with { _zod: { optout: "optional" } }, which sets the optout flag to mark the field as optional to Zod. That lets z.object infer the correct optionality.
Required<> is needed because otherwise the type's ? would be carried over to the schema itself, making it possibly undefined, which breaks z.object's inference and causes the field to degrade to unknown.
While _zod and optout is Zod's internal API, it seems to be stable and recommended to use by maintainers.

Testing

1. Testing coverage: ran pnpm run test with and without changes, number of passed tests match.
2. Tested changes on a personal project, 99+ type errors resolved.

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Closes #1494
Closes #1460