fix(Android): don't run header update for a screen detached from its stack

[nsingh-ut]

Description

ScreenStackHeaderConfig.onUpdate() and ScreenStackFragment.canNavigateBack() disagree about the same condition, so a screen detached from its stack while its header config is still attached crashes the app.

onUpdate() computes:

val stack = screenStack                                   // screen?.container as? ScreenStack
val isTop = stack == null || stack.topScreen == parent
if (!isAttachedToWindow || !isTop || isDestroyed) return

A null screenStack is treated as "is top", so execution continues. Further down it reaches:

newActionBar.setDisplayHomeAsUpEnabled(
    screenFragment?.canNavigateBack() == true && !isBackButtonHidden,
)

and canNavigateBack() asserts on exactly the state the first guard let through:

val container: ScreenContainer? = screen.container
check(container is ScreenStack) { "ScreenStackFragment added into a non-stack container" }

Both read the same Screen.container field. If it is not a ScreenStack, the permissive check passes and the strict one throws.

This is what users hit in #4429: an intermittent IllegalStateException on Android/Fabric during ordinary navigation, when a props update or an attach reaches a header config after its screen has left the stack.

Closes #4429.

Changes

• legacy/ScreenStackHeaderConfig.onUpdate() returns early when screenStack is null, instead of treating it as "is top".

Scoped to the legacy implementation deliberately: the newer stack/ code passes canNavigateBack to StackScreenFragment as a constructor value and has no equivalent assertion, so it is unaffected.

Test plan

The crash is a race and does not reproduce on demand, so I made the transient state permanent for one screen and did an A/B.

In onUpdate(), before the guard, I detached one screen from its stack. That is the same field both accessors read, so it reproduces the real end state:

if (title == "Security") {
    screen?.container = null
}

Built a debug app (RN 0.86, Expo 57, Fabric, react-native-screens 4.25.2, Android 16 emulator) and navigated to that screen.

1. Without this change: crashes every time.

java.lang.IllegalStateException: ScreenStackFragment added into a non-stack container
    at com.swmansion.rnscreens.ScreenStackFragment.canNavigateBack(ScreenStackFragment.kt:499)
    at com.swmansion.rnscreens.ScreenStackHeaderConfig.onUpdate(ScreenStackHeaderConfig.kt:268)
    at com.swmansion.rnscreens.ScreenStackHeaderConfig.onAttachedToWindow(ScreenStackHeaderConfig.kt:183)
    at android.view.View.dispatchAttachedToWindow(View.java:23105)

This matches the trace in #4429, which arrives via onAfterUpdateTransaction instead of onAttachedToWindow. Both call onUpdate(), so the guard covers both entry points.

2. With this change, detach still forced: no crash, screen renders. Its header is skipped, which is correct, since a screen with no stack has no stack header to configure.

3. With this change, detach removed: normal behaviour, title and back button render as before.

Checklist

• Included code example that can be used to test this change.
• For visual changes, included screenshots / GIFs / recordings documenting the change. (No visual change: this removes a crash.)
• For API changes, updated relevant public types. (No API change.)
• Ensured that CI passes