Fix views staying hidden when shown during their hide animation

[jan-grzybek]

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• I have read the README.
• I have read the CONTRIBUTING document and understand the caveats in the Pull Requests section.
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Commits and testing:

• My commits are rebased on the latest main branch.
• My commits are well-structured for review.
• My change has been thoroughly tested, and I am not aware of any regressions to existing features or behaviors.
• I have tested my contribution on these devices:

• iPhone 18 Pro Simulator, iOS 27.0
• iPhone 17 Pro Simulator, iOS 26.5

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Description

UIView.setIsHidden(_:using:) and setIsHidden(_:withAnimationDuration:completion:) (used by setIsHidden(_:animated:completion:)) only set isHidden once a hide animation completes. A show requested before then is compared against the stale isHidden and ignored, and the hide's completion then hides the view anyway.

Steps to reproduce on main (media review screen with a single photo):

1. Quickly tap the "view once" button twice.
2. View once is off again, but the caption field is empty and doesn't respond to taps, so no caption can be entered. The Add Media button is also missing.

The same can happen to any view using these helpers when a show and a hide are requested in quick succession.

This change tracks the most recent animated change per view (as an associated object), shared by both variants:

• A new change supersedes any pending one, including a change whose animator hasn't started yet or doesn't finish at its end (for example, one that is stopped or reversed).
• Only the most recent change's animation and completion modify the view.
• Changes without animation apply immediately.
• A show un-hides the view even if its alpha is 0. Previously it only faded alpha in and left the view hidden.

Behavior for changes that don't overlap is unchanged, including calling completion synchronously when no animation is needed. It builds on the simplification in 1a77385. Both methods now have doc comments describing this behavior.

Testing:

• Added UIKitAnimationsTest to SignalUITests: 14 tests covering both variants with real animators, waiting on completions rather than fixed delays. 8 of them fail on main: a show during a hide (animated and not, for both variants), a show before the hide's animator has started, a show during a hide while the same animator also changes the view's alpha, showing a hidden view whose alpha is 0, and a hide without animation during a hide (which leaves alpha at 0). The other 6 pass on main and guard against regressions, such as a repeated hide, or a hide whose first animator is reversed or never started.
• Reproduced the steps above on main and verified they no longer reproduce with this change. Also checked that view once toggles normally, that editing the caption, the Done button and sending still work, and that the media viewer's controls hide and show on tap (setIsHidden(_:animated:)).
• SignalUITests pass on both simulators except RecipientPickerViewControllerTests.testPhoneNumberFinderParseResults, which fails the same way on main (it depends on the device region).
• Scripts/precommit.py with SwiftFormat 0.60.1 reports no changes.

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