taprpc: add serialized anchor tx to AssetTransfer RPC message

Include the serialized anchor transaction in the AssetTransfer message
returned by the PublishAndLogTransfer RPC endpoint. This allows clients
to access the full anchor transaction used by the ChainPorter during
the transfer, enabling verification against the provided anchor PSBT.

Author: ffranr

rpcserver.go

@@ -3709,6 +3709,19 @@ func marshalOutboundParcel(
 		}
 	})
 
+	// Serialize the anchor transaction if it exists.
+	var anchorTxBytes []byte
+	if parcel.AnchorTx != nil {
+		var b bytes.Buffer
+		err := parcel.AnchorTx.Serialize(&b)
+		if err != nil {
+			return nil, fmt.Errorf("unable to serialize anchor "+
+				"tx: %w", err)
+		}
+
+		anchorTxBytes = b.Bytes()
+	}
+
 	return &taprpc.AssetTransfer{
 		TransferTimestamp:   parcel.TransferTime.Unix(),
 		AnchorTxHash:        anchorTxHash[:],
@@ -3719,6 +3732,7 @@ func marshalOutboundParcel(
 		Inputs:              rpcInputs,
 		Outputs:             rpcOutputs,
 		Label:               parcel.Label,
+		AnchorTx:            anchorTxBytes,
 	}, nil
 }
 

taprpc/assetwalletrpc/assetwallet.swagger.json

@@ -948,6 +948,11 @@
         "label": {
           "type": "string",
           "description": "An optional short label for the transfer. This label can be used to track\nthe progress of the transfer via the logs or an event subscription.\nMultiple transfers can share the same label."
+        },
+        "anchor_tx": {
+          "type": "string",
+          "format": "byte",
+          "description": "The L1 transaction that anchors the Taproot Asset commitment where the\nasset resides."
         }
       }
     },