[material_ui] Fix predictive back transition during CupertinoSheet drag

[huahua8893]

Ports flutter/flutter#187494 to material_ui following flutter/flutter#188444.

Fixes flutter/flutter#187492.

This PR fixes an issue where a route covered by a CupertinoSheet could incorrectly enter the predictive back transition during an unrelated navigator gesture, causing its corners to snap to the display corner radius while the sheet was being dragged.

PredictiveBackPageTransitionsBuilder and PredictiveBackFullscreenPageTransitionsBuilder previously used route.popGestureInProgress to determine whether to show the predictive back transition. However, this value is derived from navigator.userGestureInProgress, which becomes true for any navigator user gesture, including dragging a CupertinoSheet above the route.

The resulting corner jump is visible on physical Android devices that report display corner radii. When no display corner radius is reported, the fallback value of BorderRadius.zero masks the visual symptom. The issue does not occur on iOS because it uses CupertinoPageTransitionsBuilder.

This change uses the predictive back phase instead, so a covered route only enters the predictive back transition during an actual system back gesture. It also adds regression coverage for both predictive back transition builders.

Tests

• flutter test test/predictive_back_page_transitions_builder_test.dart --no-pub
• dart run script/tool/bin/flutter_plugin_tools.dart analyze --packages material_ui
• dart run script/tool/bin/flutter_plugin_tools.dart validate --packages material_ui --base-sha=9854cd5607da4001f2dd3cbdcd6903202d28958c --check-for-missing-changes
• dart run script/tool/bin/flutter_plugin_tools.dart publish-check --packages material_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, or I am not using AI tools.
• 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, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request updates PredictiveBackPageTransitionsBuilder and PredictiveBackFullscreenPageTransitionsBuilder to use the predictive back transition only when the transition phase is not idle, replacing the previous check on route.popGestureInProgress. This change prevents a covered route from initiating a predictive back transition while a CupertinoSheet is being dragged. A new widget test has been added to verify this behavior, along with a pending changelog entry. There are no review comments, and I have no feedback to provide.

[huahua8893]

@dkwingsmt @justinmc This is the migrated PR from flutter/flutter#187494. Could you please review the change here? Thank you!