refactor(codegen): migrate Zod/Valibot generators to thin SchemaFormatter adapters

Both generators now implement SchemaFormatter and delegate all shared logic
to SchemaCore, reducing each from ~780 lines to ~175 lines.

Also fixes a Valibot API correctness bug: non-empty required strings now
emit v.pipe(v.string(), v.minLength(1)) instead of the invalid
v.string().min(1) (Valibot does not support method chaining).

Author: directormac