add tests

ChatBot.Tests/Models/ChatSessionEntityTests.cs

@@ -1,5 +1,6 @@
 using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
+using System.Text.Json;
 using ChatBot.Models.Entities;
 using FluentAssertions;
 
@@ -634,4 +635,55 @@ public class ChatSessionEntityTests
         columnAttribute.Should().NotBeNull();
         columnAttribute!.Name.Should().Be("max_history_length");
     }
+
+    [Fact]
+    public void JsonSerialization_RoundTrip_ShouldPreserveScalarProperties()
+    {
+        // Arrange
+        var original = new ChatSessionEntity
+        {
+            Id = 101,
+            SessionId = "session-json-1",
+            ChatId = 123456789L,
+            ChatType = "group",
+            ChatTitle = "Test Group",
+            Model = "llama3.1:8b",
+            CreatedAt = DateTime.UtcNow.AddMinutes(-10),
+            LastUpdatedAt = DateTime.UtcNow.AddMinutes(-5),
+            MaxHistoryLength = 77,
+            Messages = new List<ChatMessageEntity>(),
+        };
+
+        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<ChatSessionEntity>(json, options)!;
+
+        // Assert
+        deserialized.Id.Should().Be(original.Id);
+        deserialized.SessionId.Should().Be(original.SessionId);
+        deserialized.ChatId.Should().Be(original.ChatId);
+        deserialized.ChatType.Should().Be(original.ChatType);
+        deserialized.ChatTitle.Should().Be(original.ChatTitle);
+        deserialized.Model.Should().Be(original.Model);
+        deserialized.MaxHistoryLength.Should().Be(original.MaxHistoryLength);
+        // Allow small delta for DateTime serialization
+        deserialized.CreatedAt.Should().BeCloseTo(original.CreatedAt, TimeSpan.FromSeconds(1));
+        deserialized
+            .LastUpdatedAt.Should()
+            .BeCloseTo(original.LastUpdatedAt, TimeSpan.FromSeconds(1));
+        deserialized.Messages.Should().NotBeNull();
+        deserialized.Messages.Should().BeEmpty();
+    }
 }