Fix zodOutputToConvex type inference for objects and unions

[gary-ix]

Object fields with .default() were incorrectly inferred as optional (T | undefined)
instead of required (T). This fix ensures
that zodOutputToConvex correctly uses OUTPUT types for object and union
fields, matching Zod's output type.

I ignored zod/v4, saw it was marked as WIP in the changelog.

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[Nicolapps]

Thank you for the fix!

I ignored zod/v4, saw it was marked as WIP in the changelog.

The v4 implementation is already correct:

convex-helpers/packages/convex-helpers/server/zod4.zodtoconvex.test.ts

Line 899 in 75cd216

testZodOutputToConvex(z.string().default("hello"), v.string());