Filter default gateway routes by address family in dual-stack config

[maxpain]

Summary

When a network adapter has both IPv4 and IPv6 addresses with separate default gateways in network-config v2, _process_networks picks the first route with prefixlen == 0 regardless of address family. This causes IPv6 addresses to receive the IPv4 gateway.

For example, with this config:

version: 2
ethernets:
  eth0:
    addresses:
      - 10.0.0.1/32
      - 2001:db8::1/96
    routes:
      - to: 0.0.0.0/0
        via: 169.254.0.1
      - to: ::/0
        via: fe80::1

Before: both addresses get gateway 169.254.0.1 — IPv6 default route is never configured.

After: IPv4 address gets 169.254.0.1, IPv6 address gets fe80::1.

Fix

Filter default gateway route candidates by matching the route's address family (0.0.0.0/0 for IPv4, ::/0 for IPv6) against the network address being configured.

Testing

• Added test_execute_network_details_v2_dual_stack_gateways covering dual-stack gateway selection
• Updated existing _test_execute_network_details_v2 to account for correct address family filtering
• All 22 tests pass
• Verified on Windows Server 2025 with dual-stack (IPv4 + IPv6) network configuration