Prime Bukkit material registry in JUnit 5 panel tests

The CI build for PR #397 erupted with 286 errors: `NoSuchElementException:
No value for minecraft:acacia_hanging_sign` thrown from
`org.bukkit.inventory.ItemType.<clinit>` inside `ItemStackMock.<clinit>`,
which then poisoned `ItemStackMock` for the rest of the JVM.

Root cause: ItemType's static initializer iterates Material and looks each
entry up in the registry. If it runs before MockBukkit has populated the
registry, the static init errors and every subsequent test that touches
ItemStack fails with NoClassDefFoundError. AbstractChallengesTest already
worked around this by touching `Tag.LEAVES` right after `MockBukkit.mock()`,
but seven JUnit 5 panel tests didn't extend it and didn't carry the
workaround. The flake didn't surface locally because filesystem ordering on
macOS put a primed test first; ubuntu-latest's ordering didn't.

Extract `PanelTestHelper.primeBukkitRegistry()` and call it right after
`MockBukkit.mock()` in CommonPanelTest, CommonPagedPanelTest,
ConversationUtilsTest, AdminPanelTest, ListChallengesPanelTest,
GameModePanelTest, and MultiplePanelTest. The fix is order-independent —
each affected test class now primes the registry itself.

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

Author: tastybento

src/test/java/world/bentobox/challenges/panel/CommonPagedPanelTest.java

@@ -116,6 +116,7 @@ public int getPageIndex() throws Exception {
     void setUp() {
         closeable = MockitoAnnotations.openMocks(this);
         ServerMock mbServer = MockBukkit.mock();
+        PanelTestHelper.primeBukkitRegistry();
 
         when(addon.getChallengesManager()).thenReturn(manager);
         PanelTestHelper.setupUserTranslations(user);

src/test/java/world/bentobox/challenges/panel/CommonPanelTest.java

@@ -78,6 +78,7 @@ public PanelItem getReturnButton() {
     void setUp() {
         closeable = MockitoAnnotations.openMocks(this);
         ServerMock mbServer = MockBukkit.mock();
+        PanelTestHelper.primeBukkitRegistry();
 
         when(addon.getChallengesManager()).thenReturn(manager);
         PanelTestHelper.setupUserTranslations(user);

src/test/java/world/bentobox/challenges/panel/ConversationUtilsTest.java

@@ -45,6 +45,7 @@ class ConversationUtilsTest {
     void setUp() {
         closeable = MockitoAnnotations.openMocks(this);
         ServerMock mbServer = MockBukkit.mock();
+        PanelTestHelper.primeBukkitRegistry();
 
         when(user.getTranslation(anyString())).thenAnswer(
             (Answer<String>) inv -> inv.getArgument(0, String.class));

src/test/java/world/bentobox/challenges/panel/PanelTestHelper.java

@@ -22,6 +22,23 @@
  */
 public class PanelTestHelper {
 
+    /**
+     * Force Bukkit's material registry to be populated while MockBukkit's mock
+     * server is active. Call this immediately after `MockBukkit.mock()`.
+     *
+     * <p>Why: `org.bukkit.inventory.ItemType.&lt;clinit&gt;` iterates over Material and
+     * looks each entry up in the registry. If it runs before MockBukkit has populated
+     * the registry (e.g., when a panel test that doesn't extend AbstractChallengesTest
+     * runs first on CI's filesystem ordering), it throws NoSuchElementException, and
+     * ItemStackMock stays in an errored state for the rest of the JVM — poisoning every
+     * subsequent test. Touching `Tag.LEAVES` forces the registry to populate before
+     * ItemType loads.
+     */
+    public static void primeBukkitRegistry() {
+        @SuppressWarnings("unused")
+        var unused = org.bukkit.Tag.LEAVES;
+    }
+
     /**
      * Set up user translation mocks to return the key for any invocation.
      * Uses a lenient default answer that returns the first String argument

src/test/java/world/bentobox/challenges/panel/admin/AdminPanelTest.java

@@ -54,6 +54,7 @@ class AdminPanelTest {
     void setUp() throws Exception {
         closeable = MockitoAnnotations.openMocks(this);
         ServerMock mbServer = MockBukkit.mock();
+        PanelTestHelper.primeBukkitRegistry();
 
         when(addon.getChallengesManager()).thenReturn(manager);
         PanelTestHelper.setupUserTranslations(user);

src/test/java/world/bentobox/challenges/panel/admin/ListChallengesPanelTest.java

@@ -47,6 +47,7 @@ class ListChallengesPanelTest {
     void setUp() {
         closeable = MockitoAnnotations.openMocks(this);
         ServerMock mbServer = MockBukkit.mock();
+        PanelTestHelper.primeBukkitRegistry();
 
         when(addon.getChallengesManager()).thenReturn(manager);
         PanelTestHelper.setupUserTranslations(user);

src/test/java/world/bentobox/challenges/panel/user/GameModePanelTest.java

@@ -58,6 +58,7 @@ class GameModePanelTest {
     void setUp() {
         closeable = MockitoAnnotations.openMocks(this);
         ServerMock mbServer = MockBukkit.mock();
+        PanelTestHelper.primeBukkitRegistry();
 
         when(addon.getChallengesManager()).thenReturn(manager);
         PanelTestHelper.setupUserTranslations(user);

src/test/java/world/bentobox/challenges/panel/user/MultiplePanelTest.java

@@ -53,6 +53,7 @@ class MultiplePanelTest {
     void setUp() {
         closeable = MockitoAnnotations.openMocks(this);
         ServerMock mbServer = MockBukkit.mock();
+        PanelTestHelper.primeBukkitRegistry();
 
         PanelTestHelper.setupUserTranslations(user);
         when(user.getWorld()).thenReturn(world);