fix(core): ListNode.addAll(ListNode.value()) generates a ConcurrentModificationException

Author: Siroshun09

core/src/main/java/com/github/siroshun09/configapi/core/node/ListNodeImpl.java

@@ -94,7 +94,11 @@ public void add(@Nullable Object value) {
 
     @Override
     public void addAll(@NotNull Collection<?> collection) {
-        for (var element : collection) {
+        if (collection.isEmpty()) {
+            return;
+        }
+
+        for (var element : collection.toArray()) { // We need to create an array to prevent ConcurrentModificationException
             this.backing.add(Node.fromObject(element));
         }
     }

core/src/test/java/com/github/siroshun09/configapi/core/node/ListNodeTest.java

@@ -107,6 +107,18 @@ protected Stream<NodeTestCase<ListNode>> nodeTestCases() {
                         },
                         (initial, modified) -> assertEquals(List.of(StringValue.fromString("a"), new IntValue(1), BooleanValue.TRUE), modified.value())
                 ),
+                nodeTest(
+                        "ListNode#addAll(Collection) by ListNode#value",
+                        ListNode.create(List.of("a", "b", "c")),
+                        node -> {
+                            node.addAll(node.value());
+                            return node;
+                        },
+                        (initial, modified) -> {
+                            assertEquals(6, modified.size());
+                            assertEquals(List.of("a", "b", "c", "a", "b", "c"), modified.asList(String.class));
+                        }
+                ),
                 nodeTest(
                         "ListNode#addAll(ListNode)",
                         ListNode.create(List.of("a", "b", "c")),