Complete spawner duplication exploit fix with comprehensive validation

Co-authored-by: ptthanh02 <73684260+ptthanh02@users.noreply.github.com>

Author: Copilot

core/src/main/java/github/nighter/smartspawner/commands/list/gui/adminstacker/AdminStackerHandler.java

@@ -93,6 +93,12 @@ private void handleStackChange(Player player, SpawnerData spawner, String worldN
             return;
         }
 
+        // Validate spawner integrity before allowing admin operations
+        if (spawnerManager.isGhostSpawner(spawner)) {
+            messageService.sendMessage(player, "spawner_invalid");
+            return;
+        }
+
         int newStackSize = spawner.getStackSize() + change;
 
         // Ensure stack size is within valid bounds

core/src/main/java/github/nighter/smartspawner/spawner/interactions/stack/SpawnerStackHandler.java

@@ -96,6 +96,12 @@ public boolean handleSpawnerStack(Player player, SpawnerData targetSpawner, Item
             return false;
         }
 
+        // Validate spawner integrity before allowing stacking operations
+        if (plugin.getSpawnerManager().isGhostSpawner(targetSpawner)) {
+            messageService.sendMessage(player, "spawner_invalid");
+            return false;
+        }
+
         Location location = targetSpawner.getSpawnerLocation();
         if (!hasStackPermissions(player, location)) {
             return false;