iOS: scope singleSelection to leaf header menus

[Jasleen-Kaur96]

Summary

• Fixes header menu behavior on iOS where selecting an item in one submenu clears the checkmark in a sibling submenu.
• Root cause: UIMenuOptionsSingleSelection was applied to menus that contain submenus, causing UIKit to treat sibling submenu actions as a shared selection group.
• This change scopes singleSelection to leaf menus only (menus without submenu children).

Fixes #3973.

Repro

Using unstable_headerRightItems, create a header menu with:

• an inline parent submenu
• two sibling submenus (for example Sort and Group)
• each submenu contains action items with state: 'on' | 'off'
  Observed before:
• selecting in submenu A removes checkmark in submenu B (and vice versa)
  Expected / after:
• each submenu maintains its own selected checkmark independently

Test plan

• Run app on iOS and open the header menu (...)
• Open submenu A (e.g. Sort) and select an option
• Open submenu B (e.g. Group) and select an option
• Verify both submenus keep their own checkmark state
• Swap submenu order in config and verify behavior remains correct

Environment

• iOS: 26.3
• Device: iPhone 17 Pro

[kmichalikk]

Hi @Jasleen-Kaur96
Thanks for the PR but I'm afraid that we'll not proceed here. The hasMenu checking may be incorrect in some cases. From what I can see, for native iOS having singleSelection on parent menu gets this exact behavior of selecting only one element within all child menus. We'll keep investigating but this issue will probably end up with default values change at most.