Fixing 500 error when entering a bad itemid when creating an auction

GameAuthApi.Tests/ServiceValidationTests.cs

@@ -0,0 +1,32 @@
+using GameAuthApi.Services;
+using Microsoft.Extensions.Configuration;
+
+public class ServiceValidationTests
+{
+    private static IConfiguration cfg = new ConfigurationBuilder()
+        .AddInMemoryCollection(new Dictionary<string,string>
+        {
+            {"MongoDB:ConnectionString","mongodb://localhost"},
+            {"MongoDB:DatabaseName","test"}
+        }).Build();
+
+    [Fact]
+    public async Task ItemService_InvalidId_ReturnsNull()
+    {
+        var svc = new ItemService(cfg);
+
+        var result = await svc.GetByIdAsync("bad");
+
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public async Task AuctionService_InvalidId_ReturnsNull()
+    {
+        var svc = new AuctionService(cfg);
+
+        var result = await svc.GetByIdAsync("bad");
+
+        Assert.Null(result);
+    }
+}