[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/internal/routemanager/systemops/v6route_linux_test.go

@@ -5,6 +5,7 @@ package systemops
 import (
 	"errors"
 	"net"
+	"net/netip"
 	"syscall"
 	"testing"
 
@@ -29,6 +30,7 @@ func ensureIPv6DefaultRoute(t *testing.T) {
 	}
 	if err := netlink.RouteAdd(route); err != nil {
 		if errors.Is(err, syscall.EEXIST) {
+			requireUsableIPv6Nexthop(t)
 			return
 		}
 		t.Skipf("install IPv6 fallback default route: %v", err)
@@ -38,4 +40,29 @@ func ensureIPv6DefaultRoute(t *testing.T) {
 			t.Logf("delete IPv6 fallback default route: %v", err)
 		}
 	})
+
+	requireUsableIPv6Nexthop(t)
+}
+
+// requireUsableIPv6Nexthop skips the test unless the resolved IPv6 default
+// nexthop can actually carry a route. Installing the default route succeeding
+// does not imply the kernel accepts it as a nexthop for a concrete prefix.
+func requireUsableIPv6Nexthop(t *testing.T) {
+	t.Helper()
+
+	nexthop, err := GetNextHop(netip.IPv6Unspecified())
+	if err != nil {
+		t.Skipf("resolve IPv6 default nexthop: %v", err)
+	}
+
+	probe := netip.MustParsePrefix("100::64/128")
+	switch err := addRoute(probe, nexthop, syscall.RT_TABLE_MAIN); {
+	case err == nil:
+		if err := removeRoute(probe, nexthop, syscall.RT_TABLE_MAIN); err != nil {
+			t.Logf("remove IPv6 probe route: %v", err)
+		}
+	case errors.Is(err, syscall.EEXIST):
+	default:
+		t.Skipf("IPv6 nexthop %s unusable for route installation: %v", nexthop, err)
+	}
 }