Keep the iOS pick open when a swipe down is cancelled

[vicajilau]

Fixes #2243

Problem

On iOS, starting to swipe the picker sheet down and letting it spring back completed pickFiles() with an empty list while the picker stayed open. The files selected afterwards were dropped.

presentationControllerWillDismiss finished the request right away, but UIKit calls it as soon as the user starts pulling the sheet down, even if the dismissal is then cancelled. finishCurrentRequest(nil) cleared result, so the later picker(_:didFinishPicking:) hit its guard let currentResult = result and returned early. Thanks to @chrisbobbe for the precise diagnosis.

presentationControllerWillDismiss was added in #2040 to fix #2021, where a very fast swipe could leave the picker stuck and fail every later pick with multiple_request, so it cannot simply be removed.

Change

presentationControllerWillDismiss now waits for the dismissal transition to end through the presented view controller's transitionCoordinator, and only finishes the request if the transition was not cancelled.

• Partial swipe that springs back: the transition is cancelled, the request stays open and the later selection is returned.
• Completed dismissal, including the fast swipe from [iOS] PlatformException when picker is dismissed via fast swipe — permanent stuck state until app restart #2021: the request is finished.
• No transition coordinator: the request is finished right away, as before.

presentationControllerDidDismiss keeps finishing it as well, which is harmless since finishCurrentRequest ignores a request that is already finished.

Bumps file_picker_darwin to 2.1.4.

Testing

• The example builds for the iOS simulator.
• The sheet gestures cannot be driven from tests, so this needs a manual check on the simulator: a partial swipe that springs back followed by a selection returns the file, and a fast full swipe still cancels and lets a new pick open ([iOS] PlatformException when picker is dismissed via fast swipe — permanent stuck state until app restart #2021).