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/Services/Export/BulkResultCsvExportService.cs

@@ -0,0 +1,40 @@
+using System.Globalization;
+using System.IO;
+using System.Text;
+using CsvHelper;
+using SharepointToolbox.Core.Models;
+
+namespace SharepointToolbox.Services.Export;
+
+public class BulkResultCsvExportService
+{
+    public string BuildFailedItemsCsv<T>(IReadOnlyList<BulkItemResult<T>> failedItems)
+    {
+        using var writer = new StringWriter();
+        using var csv = new CsvWriter(writer, CultureInfo.InvariantCulture);
+
+        csv.WriteHeader<T>();
+        csv.WriteField("Error");
+        csv.WriteField("Timestamp");
+        csv.NextRecord();
+
+        foreach (var item in failedItems.Where(r => !r.IsSuccess))
+        {
+            csv.WriteRecord(item.Item);
+            csv.WriteField(item.ErrorMessage);
+            csv.WriteField(item.Timestamp.ToString("o"));
+            csv.NextRecord();
+        }
+
+        return writer.ToString();
+    }
+
+    public async Task WriteFailedItemsCsvAsync<T>(
+        IReadOnlyList<BulkItemResult<T>> failedItems,
+        string filePath,
+        CancellationToken ct)
+    {
+        var content = BuildFailedItemsCsv(failedItems);
+        await System.IO.File.WriteAllTextAsync(filePath, content, new UTF8Encoding(true), ct);
+    }
+}