docs: Update to Zod v4 syntax (#84807)
### What?
Updated Zod syntax to v4 conventions in documentation examples.
### Why?
Zod 4 deprecates `message` parameter in favor of `error`, and string
format methods like `.email()` in favor of top-level functions like
`z.email()`.
### How?
- Replaced `{ message: '...' }` with `{ error: '...' }`
- Changed `z.string().email()` to `z.email()`
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Co-authored-by: Maxim Andriushin <maksim.andriushin@gmail.com>
Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>
