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/Core/Models/GraphDirectoryUser.cs

@@ -9,4 +9,5 @@ public record GraphDirectoryUser(
     string UserPrincipalName,
     string? Mail,
     string? Department,
-    string? JobTitle);
+    string? JobTitle,
+    string? UserType);