feat: Enhance room image handling by introducing default image and updating display logic; add unit tests for image resolution

DorkNet.Server.Tests/RoomServiceTests.cs

@@ -0,0 +1,39 @@
+using System.Text.Json;
+using DorkNet.Server.Data.Entities;
+using DorkNet.Server.Services;
+
+namespace DorkNet.Server.Tests;
+
+public sealed class RoomServiceTests
+{
+    [Fact]
+    public void ToWireRoom_never_emits_empty_image_name()
+    {
+        var room = new RoomEntity
+        {
+            Id = 1001,
+            Name = "CustomRoomWithoutImage",
+            Description = "Custom room",
+            ImageName = "",
+            Accessibility = 1,
+            State = 0,
+        };
+
+        var json = JsonSerializer.Serialize(RoomService.ToWireRoom(room));
+        using var document = JsonDocument.Parse(json);
+
+        Assert.Equal(RoomService.DefaultRoomImageName, document.RootElement.GetProperty("ImageName").GetString());
+    }
+
+    [Fact]
+    public void ResolveDisplayImageName_preserves_existing_image_name()
+    {
+        var room = new RoomEntity
+        {
+            Name = "Paintball",
+            ImageName = "custom-thumbnail.png",
+        };
+
+        Assert.Equal("custom-thumbnail.png", RoomService.ResolveDisplayImageName(room));
+    }
+}