[cupertino_ui] Fix covered sheet revealing root route through top gap (#12530) Ports flutter/flutter#187058 to `cupertino_ui` following flutter/flutter#188444. Fixes flutter/flutter#187057. When multiple `CupertinoSheetRoute`s are stacked, the covered sheet's top gap can reveal the root route because the top-gap padding sits outside the secondary route transition. This change applies the covered sheet's secondary transition outside the top-gap padding, so the sheet and its gap move together. It also adds a coordinate-based regression test that verifies the root route remains fully covered. ## Tests - `flutter test test/sheet_test.dart --no-pub` - `dart run script/tool/bin/flutter_plugin_tools.dart analyze --packages cupertino_ui` - `dart run script/tool/bin/flutter_plugin_tools.dart validate --packages cupertino_ui --base-sha=252bb33ad3666c7d28621c87edccafff86e210fd --check-for-missing-changes` - `dart run script/tool/bin/flutter_plugin_tools.dart publish-check --packages cupertino_ui` ## Pre-Review Checklist
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