fix: multipart bug

Author: liuyuzhe.316

volcengine-java-sdk-llmshield/src/main/java/com/volcengine/llmshield/MessageV2.java

@@ -65,4 +65,13 @@ public void setContentType(ContentTypeV2 contentType) {
     public List<MultiPart> getMultiPart() { return multiPart; }
 
     public void setMultiPart(List<MultiPart> multiPart) { this.multiPart = multiPart; }
+
+    // 追加单条历史消息
+    public MessageV2 appendMultiPart(MultiPart contPart) {
+        if (this.multiPart == null) {
+            this.multiPart = new ArrayList<>();
+        }
+        this.multiPart.add(contPart);
+        return this; // 支持链式调用
+    }
 }

volcengine-java-sdk-llmshield/src/main/java/com/volcengine/llmshield/MultiPart.java

@@ -16,6 +16,8 @@ public MultiPart(MultiPart other) {
         this.contentType = other.contentType;
     }
 
+    // 无参构造方法（保留，用于JSON反序列化等场景）
+    public MultiPart() {}
     public String getContent() { return content; }
 
     public void setContent(String content) { this.content = content; }