fix(iOS, Tabs): weak-capture tab bar item and screen view in async image callbacks

[cortinico]

Description

Avoid updating a tab bar item whose internal view hierarchy has been
torn down. This works around an iOS 26 UIKit regression where unowned
refs inside UITabBarItem._updateViewAndPositionItems: cause crashes.

Both the normal icon and selected icon completion blocks now:

1. Weak-capture tabBarItem and screenView before the block
2. Promote to strong refs inside the block and bail if either is nil
3. Guard that the screen's controller is still parented by a UITabBarController

Before & after - visual documentation

TBA

Test plan

Discussed offline

Checklist

• Included code example that can be used to test this change.
• For visual changes, included screenshots / GIFs / recordings documenting the change.
• For API changes, updated relevant public types.
• Ensured that CI passes

[kkafar]

Hello!

I can not really reproduce the issue, but it won't hurt to add a null-guard & verify whether we have been not detached from the parent controller since block-schedule-time.

One thing though -> this slightly changes semantics. Earlier, even if we were detached from the parent controller, the update would go through, effectively setting tabBarItem.image / tabBarItem.selectedImage. Now it won't. This implies that we might need to take care of reapplying the appearance at the moment we receive didMoveToParentViewController on RNSTabsScreenViewController.

I'll look into it.

[kkafar]

Okay, we already request appearance update on didMoveToParentViewController.

Seems fine then.