Fix NPE and lobby spam on unregistered peer disconnects (#10453)

Any TCP connection that drops on the game-server port before completing
Forge's registration handshake (probes, crashed handshakes, failed
rejoins) triggered an NPE in ServerGameLobby.disconnectPlayer and
broadcast "null left the lobby." to real users.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: MostCromulent

forge-gui/src/main/java/forge/gamemodes/net/server/FServerManager.java

@@ -945,11 +945,15 @@ public void run() {
                     String.format("%s disconnected. Waiting %s for reconnect...", username, formatTime(RECONNECT_TIMEOUT_SECONDS))));
                 lobbyListener.message(null, "(Host can use /skipreconnect to replace disconnected player with AI, or /skiptimeout to wait indefinitely.)");
                 netLog.info("[Disconnect] Player disconnected mid-game: {} (slot {}). Waiting for reconnect.", username, playerIndex);
-            } else {
-                // Normal disconnect (lobby or no valid slot)
+            } else if (client.hasValidSlot()) {
+                // Peer completed registration but match isn't active (or slot was freed earlier)
                 localLobby.disconnectPlayer(playerIndex);
                 broadcast(new MessageEvent(String.format("%s left the lobby.", username)));
                 broadcast(new LogoutEvent(username));
+            } else {
+                // Peer disconnected before completing registration — probe, crashed handshake, or rejection
+                netLog.info("[Disconnect] Unregistered peer disconnected from {}",
+                    ctx.channel().remoteAddress());
             }
             super.channelInactive(ctx);
         }

forge-gui/src/main/java/forge/gamemodes/net/server/ServerGameLobby.java

@@ -47,6 +47,9 @@ private void connectPlayer(final String name, final int avatarIndex, final int s
     }
     public void disconnectPlayer(final int index) {
         final LobbySlot slot = getSlot(index);
+        if (slot == null) {
+            return;
+        }
         slot.setType(LobbySlotType.OPEN);
         slot.setName(StringUtils.EMPTY);
         slot.setIsReady(false);