feat(04-01): add Phase 4 models, interfaces, BulkOperationRunner, and test scaffolds

- Install CsvHelper 33.1.0 and Microsoft.Graph 5.74.0 (main + test projects)
- Add 14 core model/enum files (BulkOperationResult, BulkMemberRow, BulkSiteRow, TransferJob, FolderStructureRow, SiteTemplate, SiteTemplateOptions, TemplateLibraryInfo, TemplateFolderInfo, TemplatePermissionGroup, ConflictPolicy, TransferMode, CsvValidationRow)
- Add 6 service interfaces (IFileTransferService, IBulkMemberService, IBulkSiteService, ITemplateService, IFolderStructureService, ICsvValidationService)
- Add BulkOperationRunner with continue-on-error and cancellation support
- Add BulkResultCsvExportService stub (compile-ready)
- Add test scaffolds: BulkOperationRunnerTests (5 passing), BulkResultCsvExportServiceTests (2 passing), CsvValidationServiceTests (6 skipped), TemplateRepositoryTests (4 skipped)

SharepointToolbox.Tests/Services/BulkResultCsvExportServiceTests.cs

@@ -0,0 +1,45 @@
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.Services.Export;
+
+namespace SharepointToolbox.Tests.Services;
+
+public class BulkResultCsvExportServiceTests
+{
+    [Fact]
+    public void BuildFailedItemsCsv_WithFailedItems_IncludesErrorColumn()
+    {
+        var service = new BulkResultCsvExportService();
+        var items = new List<BulkItemResult<BulkMemberRow>>
+        {
+            BulkItemResult<BulkMemberRow>.Failed(
+                new BulkMemberRow { Email = "bad@test.com", GroupName = "G1", GroupUrl = "http://test", Role = "Member" },
+                "User not found"),
+        };
+
+        var csv = service.BuildFailedItemsCsv(items);
+
+        Assert.Contains("Error", csv);
+        Assert.Contains("Timestamp", csv);
+        Assert.Contains("bad@test.com", csv);
+        Assert.Contains("User not found", csv);
+    }
+
+    [Fact]
+    public void BuildFailedItemsCsv_SuccessItems_Excluded()
+    {
+        var service = new BulkResultCsvExportService();
+        var items = new List<BulkItemResult<BulkMemberRow>>
+        {
+            BulkItemResult<BulkMemberRow>.Success(
+                new BulkMemberRow { Email = "ok@test.com", GroupName = "G1", GroupUrl = "http://test", Role = "Member" }),
+            BulkItemResult<BulkMemberRow>.Failed(
+                new BulkMemberRow { Email = "bad@test.com", GroupName = "G1", GroupUrl = "http://test", Role = "Member" },
+                "Error"),
+        };
+
+        var csv = service.BuildFailedItemsCsv(items);
+
+        Assert.DoesNotContain("ok@test.com", csv);
+        Assert.Contains("bad@test.com", csv);
+    }
+}