Complete the transition instead of trapping in interruptibleAnimator

[hsoi]

Thanks for #642 — this is a follow-up to it.

#641 was titled for interruptibleAnimator, but the code quoted in the report was transitionDuration, so #642 fixed the methods that were quoted. Both interruptibleAnimator methods still have the bare fatalError(), and they trap on the same condition:

guard let fpc = transitionContext.viewController(forKey: .from) as? FloatingPanelController
else { fatalError() }   // Transitioning.swift:131

When it fires

A panel is presented modally on a view controller that is itself presented modally. Dismissing the outermost presentation tears the whole stack down in one step, and UIKit then runs the panel's dismiss transition with the intermediate view controller as .from. The cast fails and the app is killed.

Impact

This is currently the single highest-volume crash in a shipping iOS app. It's a latent condition that started firing at scale without any app-side change — the same unmodified binary that had been stable for weeks began crashing, which points at an OS behaviour change rather than anything in the app. It's overwhelmingly iOS 26.5.x, but a small share of occurrences are on iOS 18.x, so it isn't iOS 26-specific.

_assertionFailure
specialized ModalDismissTransition.interruptibleAnimator(using:) (Transitioning.swift:131)
@objc ModalPresentTransition.animateTransition(using:)          <- linker-folded thunk; the dismiss path
___UIViewControllerTransitioningRunCustomTransitionWithRequest_block_invoke_3
+[UIKeyboardSceneDelegate _pinInputViewsForKeyboardSceneDelegate:onBehalfOfResponder:duringBlock:]
___UIViewControllerTransitioningRunCustomTransitionWithRequest_block_invoke_2
+[UIView(Animation) _setAlongsideAnimations:toRunByEndOfBlock:animated:]
_UIViewControllerTransitioningRunCustomTransitionWithRequest
__77-[UIPresentationController runTransitionForCurrentStateAnimated:handoffData:]_block_invoke_3

The keyboard-pinning frame appears in every sample, so a keyboard transition is consistently in flight at the same time.

The change

Both guards now return an animator that immediately completes the transition. The two nil-transitionAnimator fallbacks below them deliberately return a non-completing animator instead, because show()/hide() directly above have already scheduled completeTransition and completing twice would be worse than the trap. This also removes a force-unwrap of fpc.transitionAnimator on the dismiss path.

Worst case after this is a panel that disappears without animating, rather than a terminated process.

Tests

Added to ControllerTests, covering both guard paths, plus one pinning the existing zero-duration transitionDuration behaviour on the same condition. MockTransitionContext follows the existing MockTransitionCoordinator in TestSupports. The dismiss test also verifies completeTransition is genuinely called, since a fallback that silently never completes would be worse than the crash.

Left alone deliberately: the fatalError() in PresentationController.addFloatingPanel(), where presentedViewController is the panel by construction — happy to fold it in if you'd prefer.