ax(node): replace direct json.Unmarshal with package-local UnmarshalJSON wrapper

ParsePayload was calling encoding/json.Unmarshal directly, bypassing the
package-local UnmarshalJSON wrapper that already exists in bufpool.go.

Co-Authored-By: Charon <charon@lethean.io>

pkg/node/message.go

@@ -122,7 +122,7 @@ func (message *Message) ParsePayload(target interface{}) error {
 	if message.Payload == nil {
 		return nil
 	}
-	return json.Unmarshal(message.Payload, target)
+	return UnmarshalJSON(message.Payload, target)
 }
 
 // --- Payload Types ---