Remove CastingList from MenuFlyoutSubItem

[PureWeen]

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 splits the implementation of the internal _menu list up from the logical children. This allows Logical Children of any type to get added.

Issues Fixed

Fixes #25893

[Copilot]

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 suggestions.

Comments skipped due to low confidence (1)

src/Controls/src/Core/Menu/MenuFlyoutSubItem.cs:65

• The insertion of the item to _menus should occur after inserting it as a logical child to ensure the logical child index matches the _menus list index.

_menus.Insert(index, item);