[go_router] Fix redirect zone async error handling

[ktul15]

Fixes flutter/flutter#192066

Summary

Replaces the guarded redirect zone with a value-only zone when running redirect callbacks.

Redirect callbacks still have access to the current router through zone values, so APIs such as GoRouter.of(context) and context.namedLocation(...)
continue to work during redirects. However, unrelated unawaited async errors started during a redirect are no longer caught by go_router and wrapped as
GoException.

Tests

• fvm dart run ../../script/tool/bin/flutter_plugin_tools.dart format --packages go_router
• fvm dart run ../../script/tool/bin/flutter_plugin_tools.dart analyze --packages go_router
• fvm dart run ../../script/tool/bin/flutter_plugin_tools.dart dart-test --packages go_router
• fvm dart run ../../script/tool/bin/flutter_plugin_tools.dart validate --packages go_router
• fvm flutter test test/go_router_test.dart
• fvm flutter test test/redirect_chain_test.dart

For the checklist, check the ones that are true:

• I read the Contributor Guide...
• I read the AI contribution guidelines...
• I read the Tree Hygiene page...
• 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...
• I updated/added any relevant documentation...
• I added new tests to check the change I am making...
• All existing and new tests are passing.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gemini-code-assist]

Code Review

This pull request replaces runZonedGuarded with Zone.current.fork().run in RouteConfiguration to prevent unrelated async errors during redirects from being wrapped as GoException. It also adds a corresponding test case and a changelog entry. The reviewer noted that this change introduces a regression because synchronous exceptions thrown by the redirect callback are no longer wrapped in a GoException. They suggested wrapping the execution in a try-catch block to preserve the original error-wrapping behavior.

[gemini-code-assist]

By removing runZonedGuarded and directly returning the result of Zone.run, synchronous exceptions thrown by the redirect callback are no longer wrapped in a GoException (e.g., Exception during redirect: <error>). This is a regression in error handling behavior for synchronous redirect errors.

To preserve the wrapping of synchronous exceptions while still avoiding the issue with unrelated async errors, we can wrap the call in a try-catch block.

    try {
      return Zone.current
          .fork(zoneValues: <Object?, Object?>{currentRouterKey: router})
          .run<T>(callback);
    } catch (error) {
      throw error is GoException
          ? error
          : GoException('Exception during redirect: $error');
    }

[ktul15]

_runInRouterZone is only called from the top-level and route-level redirect handlers, and both call sites already wrap synchronous
exceptions in GoException. The top-level path wraps with Exception during redirect, while the route-level path wraps with Exception during route redirect.

The change here keeps _runInRouterZone focused on providing the router zone value only, while preserving synchronous and returned-Future redirect error
handling at the existing call sites. The added regression test covers the unrelated unawaited async error case from the issue.