test(02-03): add failing tests for SiteListService.DeriveAdminUrl

- Two tests for DeriveAdminUrl: standard URL and trailing-slash URL
- Tests fail (RED) — SiteListService not yet implemented

SharepointToolbox.Tests/Services/SiteListServiceTests.cs

@@ -0,0 +1,21 @@
+using SharepointToolbox.Services;
+using Xunit;
+
+namespace SharepointToolbox.Tests.Services;
+
+public class SiteListServiceTests
+{
+    [Fact]
+    public void DeriveAdminUrl_WithStandardUrl_ReturnsAdminUrl()
+    {
+        var result = SiteListService.DeriveAdminUrl("https://contoso.sharepoint.com");
+        Assert.Equal("https://contoso-admin.sharepoint.com", result);
+    }
+
+    [Fact]
+    public void DeriveAdminUrl_WithTrailingSlash_ReturnsAdminUrl()
+    {
+        var result = SiteListService.DeriveAdminUrl("https://contoso.sharepoint.com/");
+        Assert.Equal("https://contoso-admin.sharepoint.com", result);
+    }
+}