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/Core/Models/BulkOperationResult.cs

@@ -0,0 +1,35 @@
+namespace SharepointToolbox.Core.Models;
+
+public class BulkItemResult<T>
+{
+    public T Item { get; }
+    public bool IsSuccess { get; }
+    public string? ErrorMessage { get; }
+    public DateTime Timestamp { get; }
+
+    private BulkItemResult(T item, bool success, string? error)
+    {
+        Item = item;
+        IsSuccess = success;
+        ErrorMessage = error;
+        Timestamp = DateTime.UtcNow;
+    }
+
+    public static BulkItemResult<T> Success(T item) => new(item, true, null);
+    public static BulkItemResult<T> Failed(T item, string error) => new(item, false, error);
+}
+
+public class BulkOperationSummary<T>
+{
+    public IReadOnlyList<BulkItemResult<T>> Results { get; }
+    public int TotalCount => Results.Count;
+    public int SuccessCount => Results.Count(r => r.IsSuccess);
+    public int FailedCount => Results.Count(r => !r.IsSuccess);
+    public bool HasFailures => FailedCount > 0;
+    public IReadOnlyList<BulkItemResult<T>> FailedItems => Results.Where(r => !r.IsSuccess).ToList();
+
+    public BulkOperationSummary(IReadOnlyList<BulkItemResult<T>> results)
+    {
+        Results = results;
+    }
+}