Check for valid IPv6 host in UriComponentsBuilder.fromUriString

## Overview

PR #358 introduced a "scheme but no host" check in the `fromHttpUrl` method in `UriComponentsBuilder`, but a similar check was not added to `fromUriString`.

## Deliverables

- [x] Introduce a "scheme but no host" check in `UriComponentsBuilder.fromUriString` to align with the functionality in `fromHttpUrl`.