fix: restore accessibilityViewIsModal after nested modal dismiss

[RoyalPineapple]

Summary

When a modal is presented on top of another modal and then dismissed, the remaining modal's ContainerView is left with accessibilityViewIsModal = false. This causes VoiceOver to expose elements from both the modal and the underlying parent screen — doubling every element visible to assistive technology.

Before fix — after dismissing a nested modal:

• accessibilityViewIsModal on the remaining modal's ContainerView is false
• Parent screen elements leak through the modal mask
• VoiceOver users see duplicated elements with no way to distinguish layers

After fix:

• accessibilityViewIsModal is correctly restored to true
• Only the top modal's elements are visible to VoiceOver

Root cause

Three issues in ModalPresentationViewController:

1. Flag assignment included exiting presentations

updateAccessibilityViewIsModal() used allPresentations.map(\.containerView) which includes presentations in .pendingExit and .exiting states. The exiting presentation's ContainerView becomes .last and receives the flag, while the remaining presentation's container is set to false.

2. Early-return guard prevented re-evaluation

guard oldTopLayer != topLayer else {
    return // Flag assignment was below this — never reached
}

When the nested modal dismisses, the top accessibility layer returns to the same presentation that was on top before. The guard bails, skipping the flag assignment.

3. remove(presentation:) didn't re-evaluate the flag

After the exit animation completes, remove(presentation:) removes the presentation from allPresentations but never calls updateAccessibilityViewIsModal(). The remaining container keeps its false value permanently.

Fix

1. Move the flag assignment above the early-return guard so it runs unconditionally
2. Use presentations(includeExiting: false) to exclude exiting presentations
3. Call updateAccessibilityViewIsModal() from remove(presentation:) to restore the flag after removal

Commits

1. test: Adds a failing test that reproduces the bug — presents modal A, presents modal B on top, dismisses B, asserts accessibilityViewIsModal is true on A's ContainerView. Fails without the fix.
2. fix: Makes the test pass. 46/46 unit tests green.

Reproduction

1. Present a full-screen modal
2. From within that modal, present a second modal (any style)
3. Dismiss the second modal
4. The first modal's ContainerView now has accessibilityViewIsModal = false
5. VoiceOver can navigate to elements behind the modal that should be hidden

Test plan

• Test fails without fix (accessibilityViewIsModal = false)
• Test passes with fix (accessibilityViewIsModal = true)
• Full unit test suite passes (46/46)
• VoiceOver manual verification after nested dismiss

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