[go_router] Fix predictive back popping inactive StatefulShellBranch routes

[danyalahmed1995]

PR title

[go_router] Fix predictive back popping inactive StatefulShellBranch routes

PR body

Fixes flutter/flutter#188018

Description

This fixes Android system/predictive back handling for StatefulShellRoute.indexedStack.

Previously, when branch B was active, Android system/predictive back could still pop routes from inactive branch navigators. For example, inactive branch A and C routes could be popped while the active location remained /B1.

This change tracks whether a stateful shell branch navigator is active and prevents inactive branch navigators from participating in system/predictive back handling.

A regression test was added to verify that system/predictive back on the active branch does not pop inactive StatefulShellBranch navigators.

Issues fixed

Fixes flutter/flutter#188018

Tests

Ran from packages/go_router:

flutter test test/stateful_shell_route_system_back_test.dart
flutter test test/shell_route_system_back_test.dart
flutter test test/delegate_test.dart
flutter test test/go_router_test.dart
flutter analyze
git diff --check

Notes

No before/after screenshots are included because this is a navigation state/Android system-back behavior fix covered by a regression test.

No public API documentation update is needed because this does not add or change a public API. The behavior is covered by the added regression test.

Version/CHANGELOG note: this PR changes go_router package behavior. If the repository tooling or reviewer requires a package version and CHANGELOG.md update for this fix, I will add it before marking the PR ready/mergeable.

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.

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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 introduces a mechanism to prevent inactive StatefulShellRoute branches from being popped by the system back gesture. It adds a navigatorActive ValueListenable to the navigator builder, which is updated based on the active branch index in StatefulNavigationShellState. A new _BranchNavigatorPopScope widget wraps the branch page child with a PopScope whose canPop state is driven by this listenable. Additionally, a test is added to verify that inactive branches are not popped during back gestures. No review comments were provided.

[stuartmorgan-g]

Thanks for the contribution!

In the future, please do not delete the checklist that is in the PR template; it is there for a reason. This PR is missing required elements described in the checklist, which need to be addressed before it moves forward with review.

I am marking the PR as a Draft. Please re-add and complete the checklist, updating the PR as appropriate, and when that’s complete please feel free to mark the PR as ready for review.