add tests

ChatBot.Tests/Models/ChatMessageTests.cs

@@ -1,3 +1,4 @@
+using System.Text.Json;
 using ChatBot.Models.Dto;
 using FluentAssertions;
 using OllamaSharp.Models.Chat;
@@ -360,4 +361,24 @@ public class ChatMessageTests
         message.Content.Should().Be(content);
         message.Role.Should().Be(role);
     }
+
+    [Fact]
+    public void JsonSerialization_RoundTrip_ShouldPreserveProperties()
+    {
+        // Arrange
+        var original = new ChatMessage { Content = "Hello DTO", Role = ChatRole.Assistant };
+        var options = new JsonSerializerOptions
+        {
+            PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+            WriteIndented = false,
+        };
+
+        // Act
+        var json = JsonSerializer.Serialize(original, options);
+        var deserialized = JsonSerializer.Deserialize<ChatMessage>(json, options)!;
+
+        // Assert
+        deserialized.Content.Should().Be(original.Content);
+        deserialized.Role.Should().Be(original.Role);
+    }
 }