add tests

ChatBot.Tests/Models/ChatMessageEntityTests.cs

@@ -1,5 +1,6 @@
 using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
+using System.Text.Json;
 using ChatBot.Models.Entities;
 using FluentAssertions;
 
@@ -419,4 +420,45 @@ public class ChatMessageEntityTests
         foreignKeyAttribute.Should().NotBeNull();
         foreignKeyAttribute!.Name.Should().Be("SessionId");
     }
+
+    [Fact]
+    public void JsonSerialization_RoundTrip_ShouldPreserveScalarProperties()
+    {
+        // Arrange
+        var original = new ChatMessageEntity
+        {
+            Id = 42,
+            SessionId = 7,
+            Content = "Hello JSON",
+            Role = "user",
+            CreatedAt = DateTime.UtcNow.AddMinutes(-5),
+            MessageOrder = 3,
+            Session = null!, // ensure navigation not required for serialization
+        };
+
+        var options = new JsonSerializerOptions
+        {
+            PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+            WriteIndented = false,
+            DefaultIgnoreCondition = System
+                .Text
+                .Json
+                .Serialization
+                .JsonIgnoreCondition
+                .WhenWritingNull,
+        };
+
+        // Act
+        var json = JsonSerializer.Serialize(original, options);
+        var deserialized = JsonSerializer.Deserialize<ChatMessageEntity>(json, options)!;
+
+        // Assert
+        deserialized.Id.Should().Be(original.Id);
+        deserialized.SessionId.Should().Be(original.SessionId);
+        deserialized.Content.Should().Be(original.Content);
+        deserialized.Role.Should().Be(original.Role);
+        deserialized.MessageOrder.Should().Be(original.MessageOrder);
+        // Allow a small delta due to serialization precision
+        deserialized.CreatedAt.Should().BeCloseTo(original.CreatedAt, TimeSpan.FromSeconds(1));
+    }
 }