fix(openai): 修复无法 XML 序列化派生类的问题

src/SKIT.FlurlHttpClient.Wechat.OpenAI/Utilities/Internal/XmlUtility.cs

@@ -60,6 +60,11 @@ namespace SKIT.FlurlHttpClient.Wechat.OpenAI.Utilities
             return Serialize(typeof(T), obj);
         }
 
+        public static string Serialize(object obj)
+        {
+            return Serialize(obj.GetType(), obj);
+        }
+
         public static object Deserialize(Type type, string xml)
         {
             using var reader = new StringReader(xml);

test/SKIT.FlurlHttpClient.Wechat.OpenAI.UnitTests/TestCase_XmlUtilityTests.cs

@@ -0,0 +1,17 @@
+using Xunit;
+
+namespace SKIT.FlurlHttpClient.Wechat.OpenAI.UnitTests
+{
+    public class TestCase_XmlUtilityTests
+    {
+        [Fact(DisplayName = "测试用例：XML 序列化派生类")]
+        public void TestXmlSerializeSubClass()
+        {
+            Assert.Null(Record.Exception(() =>
+            {
+                object obj = new Models.SignRequest() { UserId = "TEST" };
+                Utilities.XmlUtility.Serialize(obj);
+            }));
+        }
+    }
+}