fix(valibot): apply directives to list fields instead of their items

[Kretiss]

Follow-up to #489 / #1423, which fixed this for the Zod generators.

The valibot generator still applies a list field's directives to each item, so a length constraint on an array
limits every element instead of the array:

input UserCreateInput {
  tags: [String!]! @constraint(minLength: 1, maxLength: 10)
}

// before
tags: v.array(v.pipe(v.string(), v.minLength(1), v.maxLength(10)))
// after
tags: v.pipe(v.array(v.string()), v.minLength(1), v.maxLength(10))

This ports the isRoot handling of generateFieldTypeZodSchema to generateFieldTypeValibotSchema: the directives
go on the outermost v.array(...) of the field (non-null and nullable lists alike), and items get none. Scalar fields
are unchanged, and notAllowEmptyString still applies to string items.

The it.todo('list field') under "issues #19" is now a real test covering [String!]!, [String!] and a nested
[[String!]!]!.

🤖 Generated with Claude Code