[cupertino_ui] Prevent intercepting CupertinoSheet transitions mid-animation

[xxxOVALxxx]

Fixed an incompatibility between CupertinoSheetRoute and SwiftUI sheet, which allowed users to intercept animations for opening and closing a CupertinoSheetRoute, resulting in unexpected gesture behavior and animation failures.

This PR is a follow-up to flutter/flutter#181605

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[gemini-code-assist]

Code Review

This pull request prevents drag gestures from interrupting the Cupertino sheet's dismissal animation by updating enabledCallback to check if the route controller is currently animating, and adding a check for enabledCallback in _CupertinoDraggableScrollableSheetState._dragStart. A widget test is also added to verify this behavior. The reviewer suggests also checking widget.enabledCallback() in _handleDragStart within _CupertinoDragGestureDetectorState to handle cases where an animation starts after a pointer is downed but before the drag gesture begins.

[gemini-code-assist]

While enabledCallback is now correctly updated to check if the route is animating, _CupertinoDragGestureDetectorState only checks enabledCallback() during onPointerDown (line 1059).

If a programmatic animation starts after the pointer is downed but before the drag gesture actually starts (i.e., before the pointer moves enough to trigger onStart), _handleDragStart will still be called and will initiate the pop gesture, potentially intercepting the animation.

To make this fully robust and consistent with _CupertinoDraggableScrollableSheetState (which checks enabledCallback in _dragStart), consider also checking widget.enabledCallback() in _handleDragStart and handling a null _dragGestureController in _handleDragUpdate and _handleDragEnd.

[xxxOVALxxx]

Added in the following PR #12516