test(14-01): add failing tests for SelectDirectoryUserCommand

- Test 17: adds user to SelectedUsers
- Test 18: skips duplicates
- Test 19: null does nothing
- Test 20: user is auditable after selection

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

SharepointToolbox.Tests/ViewModels/UserAccessAuditViewModelDirectoryTests.cs

@@ -348,4 +348,59 @@ public class UserAccessAuditViewModelDirectoryTests
         Assert.Single(visible);
         Assert.Equal("Alice", visible[0].DisplayName);
     }
+
+    // ── Test 17: SelectDirectoryUserCommand adds user to SelectedUsers ──────
+
+    [Fact]
+    public void SelectDirectoryUserCommand_adds_user_to_SelectedUsers()
+    {
+        var (vm, _, _) = CreateViewModel();
+        var dirUser = MakeMember("Alice");
+
+        vm.SelectDirectoryUserCommand.Execute(dirUser);
+
+        Assert.Single(vm.SelectedUsers);
+        Assert.Equal("Alice", vm.SelectedUsers[0].DisplayName);
+        Assert.Equal("alice@contoso.com", vm.SelectedUsers[0].UserPrincipalName);
+    }
+
+    // ── Test 18: SelectDirectoryUserCommand skips duplicates ─────────────────
+
+    [Fact]
+    public void SelectDirectoryUserCommand_skips_duplicates()
+    {
+        var (vm, _, _) = CreateViewModel();
+        var dirUser = MakeMember("Alice");
+
+        vm.SelectDirectoryUserCommand.Execute(dirUser);
+        vm.SelectDirectoryUserCommand.Execute(dirUser);
+
+        Assert.Single(vm.SelectedUsers);
+    }
+
+    // ── Test 19: SelectDirectoryUserCommand with null does nothing ───────────
+
+    [Fact]
+    public void SelectDirectoryUserCommand_with_null_does_nothing()
+    {
+        var (vm, _, _) = CreateViewModel();
+
+        vm.SelectDirectoryUserCommand.Execute(null);
+
+        Assert.Empty(vm.SelectedUsers);
+    }
+
+    // ── Test 20: After SelectDirectoryUser, user can be audited ──────────────
+
+    [Fact]
+    public void SelectDirectoryUser_adds_auditable_user_to_SelectedUsers()
+    {
+        var (vm, _, _) = CreateViewModel();
+        var dirUser = MakeMember("Alice");
+
+        vm.SelectDirectoryUserCommand.Execute(dirUser);
+
+        Assert.True(vm.SelectedUsers.Count > 0);
+        Assert.Equal("alice@contoso.com", vm.SelectedUsers[0].UserPrincipalName);
+    }
 }