fix(graph): avoid mutating append inputs

Author: Gujiassh

spring-ai-alibaba-graph-core/src/main/java/com/alibaba/cloud/ai/graph/state/strategy/AppendStrategy.java

@@ -94,7 +94,9 @@ else if (newValue instanceof Collection) {
 				oldList.addAll(list);
 			}
 			else {
-				oldList.add(newValue);
+				var result = new ArrayList<>(oldList);
+				result.add(newValue);
+				return result;
 			}
 			return oldList;
 		}

spring-ai-alibaba-graph-core/src/test/java/com/alibaba/cloud/ai/graph/AppendStrategyMutationRegressionTest.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2024-2026 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.alibaba.cloud.ai.graph;
+
+import com.alibaba.cloud.ai.graph.state.strategy.AppendStrategy;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotSame;
+
+class AppendStrategyMutationRegressionTest {
+
+    // https://github.com/alibaba/spring-ai-alibaba/issues/4757
+    @Test
+    void issue4757_singleValueAppend_doesNotMutateExistingList() {
+        AppendStrategy strategy = new AppendStrategy();
+        List<Object> oldValues = new ArrayList<>(List.of("question"));
+
+        Object result = strategy.apply(oldValues, "user-answer");
+
+        assertEquals(List.of("question"), oldValues);
+        assertEquals(List.of("question", "user-answer"), result);
+        assertNotSame(oldValues, result);
+    }
+}