feat(13-01): extend GraphDirectoryUser with UserType and add includeGuests parameter to directory service

- Add string? UserType as last positional parameter to GraphDirectoryUser record
- Add bool includeGuests = false parameter to IGraphUserDirectoryService.GetUsersAsync
- Branch Graph filter: members-only (default) vs all users when includeGuests=true
- Add userType to Graph Select array for MapUser population
- Update MapUser to include UserType from Graph User object
- Add MapUser_PopulatesUserType and MapUser_NullUserType tests

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

SharepointToolbox/Services/IGraphUserDirectoryService.cs

@@ -13,6 +13,10 @@ public interface IGraphUserDirectoryService
     /// Iterates through all pages using the Graph SDK PageIterator until exhausted or cancelled.
     /// </summary>
     /// <param name="clientId">The client/tenant identifier used to obtain a Graph token.</param>
+    /// <param name="includeGuests">
+    /// When <c>false</c> (default), only member users are returned (userType eq 'Member').
+    /// When <c>true</c>, both members and guests are returned (no userType filter).
+    /// </param>
     /// <param name="progress">
     /// Optional progress reporter — receives the running count of users fetched so far.
     /// Phase 13's ViewModel uses this to show "Loading... X users" feedback.
@@ -21,6 +25,7 @@ public interface IGraphUserDirectoryService
     /// <param name="ct">Cancellation token. Iteration stops when cancelled.</param>
     Task<IReadOnlyList<GraphDirectoryUser>> GetUsersAsync(
         string clientId,
+        bool includeGuests = false,
         IProgress<int>? progress = null,
         CancellationToken ct = default);
 }