Fix equipment slot and group API

Adds the following:
- Add test for EquipmentSlotGroup
- Expose LivingEntity#canUseSlot

Co-authored-by: SoSeDiK <mrsosedik@gmail.com>

paper-server/src/main/java/org/bukkit/craftbukkit/entity/CraftLivingEntity.java

@@ -1230,4 +1230,11 @@ public class CraftLivingEntity extends CraftEntity implements LivingEntity {
         this.getHandle().setYBodyRot(bodyYaw);
     }
     // Paper end - body yaw API
+
+    // Paper start - Expose canUseSlot
+    @Override
+    public boolean canUseEquipmentSlot(org.bukkit.inventory.EquipmentSlot slot) {
+        return this.getHandle().canUseSlot(org.bukkit.craftbukkit.CraftEquipmentSlot.getNMS(slot));
+    }
+    // Paper end - Expose canUseSlot
 }

paper-server/src/main/java/org/bukkit/craftbukkit/inventory/CraftInventoryPlayer.java

@@ -135,6 +135,10 @@ public class CraftInventoryPlayer extends CraftInventory implements org.bukkit.i
             case HEAD:
                 this.setHelmet(item);
                 break;
+                // Paper start
+            case BODY:
+                throw new IllegalArgumentException("BODY is not valid for players!");
+                // Paper end
             default:
                 throw new IllegalArgumentException("Could not set slot " + slot + " - not a valid slot for PlayerInventory");
         }
@@ -162,6 +166,10 @@ public class CraftInventoryPlayer extends CraftInventory implements org.bukkit.i
                 return java.util.Objects.requireNonNullElseGet(this.getChestplate(), () -> new ItemStack(org.bukkit.Material.AIR)); // Paper - make nonnull
             case HEAD:
                 return java.util.Objects.requireNonNullElseGet(this.getHelmet(), () -> new ItemStack(org.bukkit.Material.AIR)); // Paper - make nonnull
+                // Paper start
+            case BODY:
+                throw new IllegalArgumentException("BODY is not valid for players!");
+                // Paper end
             default:
                 throw new IllegalArgumentException("Could not get slot " + slot + " - not a valid slot for PlayerInventory");
         }

paper-server/src/main/java/org/bukkit/craftbukkit/inventory/CraftMetaItem.java

@@ -1723,7 +1723,7 @@ class CraftMetaItem implements ItemMeta, Damageable, Repairable, BlockDataMeta {
         if (this.attributeModifiers == null) return LinkedHashMultimap.create(); // Paper - don't change the components
         SetMultimap<Attribute, AttributeModifier> result = LinkedHashMultimap.create();
         for (Map.Entry<Attribute, AttributeModifier> entry : this.attributeModifiers.entries()) {
-            if (entry.getValue().getSlot() == null || entry.getValue().getSlot() == slot) {
+            if (entry.getValue().getSlotGroup().test(slot)) { // Paper - correctly test slot against group
                 result.put(entry.getKey(), entry.getValue());
             }
         }
@@ -1797,9 +1797,7 @@ class CraftMetaItem implements ItemMeta, Damageable, Repairable, BlockDataMeta {
 
         while (iter.hasNext()) {
             Map.Entry<Attribute, AttributeModifier> entry = iter.next();
-            // Explicitly match against null because (as of MC 1.13) AttributeModifiers without a -
-            // set slot are active in any slot.
-            if (entry.getValue().getSlot() == null || entry.getValue().getSlot() == slot) {
+            if (entry.getValue().getSlotGroup().test(slot)) { // Paper - correctly test slot against group
                 iter.remove();
                 ++removed;
             }