multi: add ChanEdgePolicyFromWire constructor for ChannelEdgePolicy

Add ChanEdgePolicyFromWire to construct ChannelEdgePolicy from channel
update messages, centralizing v1/v2 field mapping.
Update call sites to use the helper:

- discovery/gossiper: handleChanUpdate
- graph/builder: ApplyChannelUpdate
- routing/router_test: ApplyChannelUpdate test helper

This consolidates update-to-policy conversion logic across versions.

routing/router_test.go

@@ -2968,20 +2968,13 @@ func (m *mockGraphBuilder) ApplyChannelUpdate(msg *lnwire.ChannelUpdate1) bool {
 		return false
 	}
 
-	err := m.updateEdge(&models.ChannelEdgePolicy{
-		Version:                   msg.GossipVersion(),
-		SigBytes:                  msg.Signature.ToSignatureBytes(),
-		ChannelID:                 msg.ShortChannelID.ToUint64(),
-		LastUpdate:                time.Unix(int64(msg.Timestamp), 0),
-		MessageFlags:              msg.MessageFlags,
-		ChannelFlags:              msg.ChannelFlags,
-		TimeLockDelta:             msg.TimeLockDelta,
-		MinHTLC:                   msg.HtlcMinimumMsat,
-		MaxHTLC:                   msg.HtlcMaximumMsat,
-		FeeBaseMSat:               lnwire.MilliSatoshi(msg.BaseFee),
-		FeeProportionalMillionths: lnwire.MilliSatoshi(msg.FeeRate),
-		ExtraOpaqueData:           msg.ExtraOpaqueData,
-	})
+	update, err := models.ChanEdgePolicyFromWire(
+		msg.ShortChannelID.ToUint64(), msg,
+	)
+	if err != nil {
+		return false
+	}
+	err = m.updateEdge(update)
 
 	return err == nil
 }