Fix CI: initialize MockBukkit in InventoryStorageTest

Paper API 1.21.11 routes ItemStack constructors through ItemStack.of()
which requires Bukkit.server to be set. InventoryStorageTest was creating
ItemStack objects without MockBukkit initialization, causing class init
failure that cascaded to all other test classes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: tastybento

src/test/java/com/wasteofplastic/invswitcher/dataobjects/InventoryStorageTest.java

@@ -16,10 +16,12 @@
 import org.bukkit.Statistic;
 import org.bukkit.entity.EntityType;
 import org.bukkit.inventory.ItemStack;
+import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
 import org.mockito.junit.jupiter.MockitoExtension;
+import org.mockbukkit.mockbukkit.MockBukkit;
 
 /**
  * @author tastybentos
@@ -35,9 +37,15 @@ public class InventoryStorageTest {
      */
     @BeforeEach
     public void setUp() {
+        MockBukkit.mock();
         is = new InventoryStorage();
     }
 
+    @AfterEach
+    public void tearDown() {
+        MockBukkit.unmock();
+    }
+
     /**
      * Test method for {@link com.wasteofplastic.invswitcher.dataobjects.InventoryStorage#getUniqueId()}.
      */