fix(dns): support bare IPv6 resolver addresses

[Harshal96]

Motivation:

Fixes #6194

Fixes parsing of configured DNS resolver addresses so bare IPv6 literals are accepted without being mistaken for host:port values.

IPv6 DNS server addresses contain : characters, so the previous lastIndexOf(':') parsing treated bare IPv6 literals as if they included a port. That could reject valid resolver addresses or parse them incorrectly.

How

• Detects whether a configured DNS server string actually includes a port.
• Uses Vert.x URI parsing helpers to distinguish IP literals from host:port values.
• Preserves support for IPv6 resolver addresses that explicitly include a port.
• Adds regression coverage for bare IPv6 resolver addresses and IPv6 addresses with ports.

Testing

• Added NameResolverTest coverage for bare IPv6 DNS servers.
• Added coverage for IPv6 DNS server entries with explicit ports.