[Windows] Fix crash setting MenuFlyoutSubItem IconImageSource

[jsuarezruiz]

Description of Change

Fix crash setting MenuFlyoutSubItem IconImageSource on Windows.

From #24688 the ImageSource is added as logical children of the menu item. However, the LogicalChildrenInternalBackingStore collection from MenuFlyoutSubItem does a casting to IMenuElement

maui/src/Controls/src/Core/Menu/MenuFlyoutSubItem.cs

Line 19 in 5b5aaa5

LogicalChildrenInternalBackingStore = new CastingList<Element, IMenuElement>(_menus);

Trying to add the IconImageSource to the LogicalChildrenInternalBackingStore insert a null value because this casting

maui/src/Controls/src/Core/ReadOnlyCastingList.cs

Line 91 in 5b5aaa5

_list.Add(item as TFrom);

fails, cannot convert from Element to IMenuElement.

This PR apply changes to allow Element in the LogicalChildrenInternalBackingStore collection from MenuFlyoutSubItem.

Issues Fixed

Fixes #25893

[PureWeen]

Can you use a unit test here instead of a UI test?

[jsuarezruiz]

Can you use a unit test here instead of a UI test?

Added both, xlml test and UITest.

[PureWeen]

I don't know if this will work...

I feel like we've hit a point on this one where we actually need to maintain two different internal lists.

_menus no longer represents _menus it's a mix of types.

If someone calls RemoveAt(index) they are going to expect that it removes the menu specifically at the given index (we should also make sure this removes the ImageSource added)

So, I think we have to keep _menus as a list of IMenuElement to satisfy all the IList<IMenuElement> implementations.

And then the backing store for logical children basically has to be a mix of both.

We basically do this with Layout.cs, it has an internal list of _children it uses to satisfy the direct children but then it adds each of this to the logical children on the parent

[PureWeen]

#26558