This PR fixes delays when IPv6 DNS lookups stall otherwise working IPv4 connections.
The earlier changes ([04c74877](04c74877c3) and [8a6f7784](8a6f778410)) added fallback across available IPv4 and IPv6 addresses, which this PR preserves.
Resolving both families upfront appears to have been an implementation choice, yet now IPv6 is resolved only when no IPv4 connection succeeds.
Unit tests and real socket compatibility tests passed on Alma 10 and Debian 13.
