Add JsonConstructor specs to allow for correct deserialization of readonly fields

osu.Game/Online/RealtimeMultiplayer/MultiplayerRoom.cs

@@ -5,6 +5,7 @@
 
 using System;
 using System.Collections.Generic;
+using Newtonsoft.Json;
 
 namespace osu.Game.Online.RealtimeMultiplayer
 {
@@ -41,6 +42,7 @@ public class MultiplayerRoom
 
         private object writeLock = new object();
 
+        [JsonConstructor]
         public MultiplayerRoom(in long roomId)
         {
             RoomID = roomId;

osu.Game/Online/RealtimeMultiplayer/MultiplayerRoomUser.cs

@@ -4,6 +4,7 @@
 #nullable enable
 
 using System;
+using Newtonsoft.Json;
 using osu.Game.Users;
 
 namespace osu.Game.Online.RealtimeMultiplayer
@@ -17,6 +18,7 @@ public class MultiplayerRoomUser : IEquatable<MultiplayerRoomUser>
 
         public User? User { get; set; }
 
+        [JsonConstructor]
         public MultiplayerRoomUser(in int userId)
         {
             UserID = userId;