[Revert] [iOS] CollectionView with grouped data crashes on iOS when the groups change - fix

[PureWeen]

Description of Change

This caused a regression for SR5 so we are going to revert for now and fix the original for SR6.

Issues Fixed

Fixes #28098

[Copilot]

PR Overview

This PR reverts a regression fix for iOS that caused a blank screen when navigating back to MainPage and restores the original behavior for SR6. Key changes include:

• Adding new test cases (Issue28098) in both Shared.Tests and HostApp to validate the restored behavior.
• Updating the iOS handler in ObservableItemsSource by removing the collectionView.Window null check.
• Removing group index boundary checks in ObservableGroupedSource and deleting the old Issue27797 tests.

Reviewed Changes

File	Description
src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue28098.cs	New test case validating the blank screen issue on navigation back.
src/Controls/tests/TestCases.HostApp/Issues/Issue28098.xaml.cs	New UI implementation supporting the Issue28098 test.
src/Controls/src/Core/Handlers/Items/iOS/ObservableItemsSource.cs	Removed the check for collectionView.Window being null.
src/Controls/src/Core/Handlers/Items/iOS/ObservableGroupedSource.cs	Removed boundary checks on group index values.
src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue27797.cs	Removed redundant tests related to the previous regression.
src/Controls/tests/TestCases.HostApp/Issues/Issue27797.xaml.cs	Removed redundant host app tests for the previous issue.

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

src/Controls/src/Core/Handlers/Items/iOS/ObservableItemsSource.cs:317

• Removing the check for collectionView.Window being null may lead to UI updates being attempted on a collectionView that is not attached to a window. Verify that this change does not introduce race conditions or unexpected behavior when the view is not fully loaded.

if (collectionView.Hidden)

src/Controls/src/Core/Handlers/Items/iOS/ObservableGroupedSource.cs:315

• The removal of the boundary check for the groupIndex might lead to an IndexOutOfRangeException if groupIndex is not within a valid range. Please ensure that groupIndex is always valid before accessing _groupSource.

switch (_groupSource[groupIndex])

[PureWeen]

/rebase

[PureWeen]

/rebase