Add end-to-end test cases to TestUnmarshalPolicy that verify bracketed
IPv6 addresses are correctly parsed through the full policy pipeline
(JSON unmarshal -> splitDestinationAndPort -> parseAlias -> parsePortRange)
and survive JSON round-trips.
Cover single port, multiple ports, wildcard port, CIDR prefix, port
range, bracketed IPv4, and hostname rejection.
Updates #2754
b668c7a596