[cupertino_ui] Fix covered sheet revealing root route through top gap

[huahua8893]

Ports flutter/flutter#187058 to cupertino_ui following flutter/flutter#188444.

Fixes flutter/flutter#187057.

When multiple CupertinoSheetRoutes 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

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets.
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions and added a pending patch changelog.
• I updated or added relevant documentation.
• I added a regression test for this change.
• All relevant existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request modifies the widget tree in _CupertinoSheetTransitionState to wrap the Padding widget with _coverSheetSecondaryTransition instead of having Padding wrap the transition. This prevents lower routes from being revealed through the top gap when multiple CupertinoSheetRoutes are stacked. A new widget test has been added to verify this layout behavior, and a pending changelog entry has been created. There are no review comments to evaluate.

[huahua8893]

@dkwingsmt @victorsanni This is the migrated PR from flutter/flutter#187058. Could you please review the change here? Thank you!