fix: support IPv6 connection monitor targets

[itsDNNS]

Summary

• Adds IPv6 support to Connection Monitor probing for IPv6 literals, IPv6-only hostnames, and dual-stack targets.
• Updates TCP probing to resolve with AF_UNSPEC and try usable IPv4/IPv6 addresses instead of assuming IPv4.
• Extends the setuid ICMP helper to support ICMPv6 and dual-stack fallback while preserving the existing icmp/tcp probe method values.
• Adds regression coverage for IPv6 TCP/ICMP behavior, clean DNS failures, dual-stack fallback, and helper timeout planning.

Tests

• pytest tests/modules/connection_monitor -q - 172 passed
• gcc -O2 -Wall -Werror -o /tmp/docsight-icmp-helper-test tools/icmp_probe_helper.c
• git diff --check origin/main

Closes #361