Add utility function for pretty printing validation errors like in Zod v4

[mintydev789]

I know there is the flattenValidationErrors function, but it still returns an object with per-field validation errors. Zod 4 came out with the z.prettifyError() function, which provides very nice pretty printing of validation errors. It would be nice for us to have something like that that returns a simple string too, because otherwise we still have to go through the flattened validation errors objects to condense them into strings for display.