EntityCollection fixes: IList indexer should not return null, update INotifyCollectionChanged index calculation

[Copilot]

The previous EntityCollection CollectionView regressions did not exercise the reported failure shapes. This update swaps them for targeted scenarios that hit sorted-view update paths and null-item filter behavior, and mirrors the same coverage in Avalonia.

• WPF regression suite (EntityCollectionTests.CollectionViewTests)

  • Replaced broad sorted/filtered/grouped tests with focused cases:
    • SortedView_EntitySetClear_DoesNotThrowNRE
    • SortedView_EntityCollectionAdd_DoesNotThrowNRE
    • SortedView_SourceSetRemove_DoesNotThrowNRE
    • FilteredView_FilterPredicate_NeverInvokedWithNull
  • Uses ListCollectionView + SortDescriptions to force PrepareLocalArray/AdjustBefore-style paths that match the reported behavior.

• Avalonia parity (EntityCollectionTests.AvaloniaTests)

  • Added equivalent regression coverage using DataGridCollectionView for the same four scenarios.
  • Keeps expected behavior aligned across WPF and Avalonia collection-view integrations.

• Small cleanup

  • Removed redundant entitySet = null initialization in one WPF test.

ListCollectionView view = (ListCollectionView)((ICollectionViewFactory)entityCollection).CreateView();
view.SortDescriptions.Add(new SortDescription(nameof(City.Name), ListSortDirection.Ascending));
view.Refresh();

Exception caught = null;
try { entitySet.Clear(); } catch (Exception e) { caught = e; }

Assert.IsNull(caught, $"EntitySet.Clear() must not surface NRE through the bound view: {caught}");

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
78.9% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud