Test IPv6 and dual-stack entrypoints

[dkropachev]

Jira: DRIVER-823

Cover IPv6 and dual-stack discovery behavior with real Apache transport connections:

• exercise a raw IPv6 literal seed and learned raw IPv6 node
• assert the bracketed IPv6 URI and Host header used for routing
• cover deterministic A-only and AAAA-only DNS entrypoints
• cover failed-AAAA-to-A and failed-A-to-AAAA fallback using an injected Apache DNS resolver
• verify all-unavailable dual-stack discovery returns promptly and preserves the configured seed

The existing implementation already keeps configured seeds as recovery candidates, replaces routing seeds with learned nodes after successful discovery, handles partial cluster refresh, and traverses scope fallback chains.

Verification: full lint (license, format, checkstyle, compile, and javadocs), all 786 unit tests on Java 21, and the new transport tests on Java 11.