Prevent iOS CollectionView size shifts from clearing the cell size cache

[hartez]

Description of Change

In some circumstances, the iOS CollectionView runs into an issue where rounding/floating point math differences cause the UICollectionViewLayout to trigger a bounds-change layout incorrectly. This can happen continuously during scrolling operations, which causes the cell size cache to repeatedly clear, which in turn causes constant cell resizing and shifting.

These changes eliminate one source of diverging size information and widen the tolerance for other layout triggers, preventing the continuous cache clearing. This also introduces an automated test to ensure the cell size cache is active.

Issues Fixed

Fixes #17890

[maonaoda]

It seems that this problem also exists on other view on iOS.

When I tried to solve this #17885, I found that there was always a very small gap in the size before and after, although the size did not change.

    public class HeaderFooterLayoutView : LayoutView
    {
        public override void LayoutSubviews()
        {
            base.LayoutSubviews();

            var bounds = AdjustForSafeArea(Bounds).ToRectangle();

            var widthConstraint = bounds.Width;
            var heightConstraint = bounds.Height;

            var size = CrossPlatformLayout.CrossPlatformMeasure(widthConstraint, double.PositiveInfinity);
            if (CrossPlatformLayout is VisualElement element
                && element.Parent is StructuredItemsView
                && (Math.Round(heightConstraint, 3, MidpointRounding.ToZero) != Math.Round(size.Height, 3, MidpointRounding.ToZero)))
            {
                element.InvalidateMeasureNonVirtual(Microsoft.Maui.Controls.Internals.InvalidationTrigger.Undefined);
            }
        }
    }

[Redth]

/rebase