ensureIPv6DefaultRoute treated a successful netlink RouteAdd as proof that
a usable IPv6 nexthop exists. Installing ::/0 via loopback can succeed while
the kernel still rejects that nexthop for a concrete prefix, which surfaced
on ubuntu22/20260810.260 runners as:
add route to table: netlink add route: invalid argument
Probe the resolved nexthop by installing and removing a discard-prefix route
through the same code path the tests use, and skip when it fails. EEXIST
means the nexthop already carries a route, so it counts as usable.
7bab15fa42