* [client] Skip IPv6 route tests when the default nexthop is unusable
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.
* [client] Probe the IPv6 nexthop through raw netlink
addRoute swallows EAFNOSUPPORT and EOPNOTSUPP via isOpErr, so a nil return
did not prove the probe route was installed. Call netlink directly so an
unsupported operation skips the test instead of passing as usable.
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