Treat a path with AS 0 anywhere as a withdrawal (closes #141)

An AS path that holds AS 0 anywhere, a member of an AS set included, now
decodes as an empty path. RFC 7607 makes such a path malformed and its
announcements withdrawals. An empty path already gets that handling:
the prefix handler deletes the peer's route for the prefix, and the AS
and peering handlers record nothing from it. Before, a path such as
[64496, 0, 64497] stored the route with origin AS64497, recorded both
ASes, and recorded the peerings between the path's other ASes.

Model: opus-5-5

internal/routewatch/peeringhandler_internal_test.go

@@ -153,6 +153,43 @@ func TestPeeringHandlerLeavesATrailingASSetOutOfThePeerings(t *testing.T) {
 	}
 }
 
+// TestPeeringHandlerRecordsNothingFromAPathWithASZero handles AS paths that
+// hold AS 0. RFC 7607 makes such a path malformed, so no peering is recorded
+// from it, not even one between two other ASes of the path.
+func TestPeeringHandlerRecordsNothingFromAPathWithASZero(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		path []any
+	}{
+		{name: "AS 0 between two ASes", path: []any{testASNA, 0, testASNB}},
+		{
+			name: "AS 0 after two ASes",
+			path: []any{testASNA, testASNB, 0, testASNC},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			store := newMockStore()
+			h := newTestHandler(store)
+			h.HandleMessage(&ristypes.RISMessage{
+				Path:            decodePath(t, tt.path...),
+				ParsedTimestamp: time.Now().UTC(),
+			})
+			h.ProcessPeeringsNow()
+
+			if len(store.Peerings) > 0 {
+				t.Errorf("peerings recorded = %v, want none",
+					slices.Sorted(maps.Keys(store.Peerings)))
+			}
+		})
+	}
+}
+
 // TestPeeringHandlerProcessesEachRunAndEmpties verifies that a processing run
 // empties the path map (the swap) and that a path seen again after a run is
 // recorded in the next run too.