refactoring

wizard-common/src/main/java/eu/jonahbauer/wizard/common/messages/server/SessionJoinedMessage.java

@@ -34,5 +34,16 @@ public final class SessionJoinedMessage extends ServerMessage implements Respons
         this.player = player;
         this.players = List.copyOf(players);
         this.secret = secret;
+
+        boolean found = false;
+        for (var playerData : players) {
+            if (playerData.getUuid().equals(player)) {
+                found = true;
+                break;
+            }
+        }
+        if (!found) {
+            throw new IllegalArgumentException("Player not contained in players.");
+        }
     }
 }