routing: add logging for routes discarded due to low success probability

Create the ChanIDString function to return a string representation
of the route's channel IDs, formatting them in the order they appear
in the route (e.g., "chanID1 -> chanID2").

Discarded routes with a success probability lower than the minimum
threshold are now logged accordingly when finding a blinded path.

routing/route/route.go

@@ -773,3 +773,19 @@ func (r *Route) String() string {
 		b.String(), r.TotalTimeLock,
 	)
 }
+
+// ChanIDString returns the route's channel IDs as a formatted string.
+func ChanIDString(r *Route) string {
+	var b strings.Builder
+
+	for i, hop := range r.Hops {
+		b.WriteString(fmt.Sprintf("%v",
+			strconv.FormatUint(hop.ChannelID, 10),
+		))
+		if i != len(r.Hops)-1 {
+			b.WriteString(" -> ")
+		}
+	}
+
+	return b.String()
+}