[go_router] Fixes StatefulShellRoute PopScope behavior to respect back button

[chunhtai]

Fixes issue #181945.

Root Cause

StatefulShellRoute manages multiple branches, each with its own parallel navigation stack. When the system back button is pressed on a non-root branch (index > 0), the framework does not inherently know that it should oscillate back to the root branch (index 0) when the current branch's navigator cannot pop further. This caused system back events to exit the app or behave inconsistently.

Fix

This PR adds a PopScope wrapper around the StatefulNavigationShell container:

1. Conditional Blocking: It sets canPop: widget.currentIndex == 0. This allows natural app exiting only from the root branch.
2. Branch Oscillation: When a pop is requested on a non-root branch, it is blocked, and the onPopInvokedWithResult handler calls goBranch(0) to switch back to the main tab.
3. Compatibility: Inner PopScope widgets placed within specific branches will still function for pops within their respective nested navigators.

[chunhtai]

@gemini-code-assist review

[gemini-code-assist]

Code Review

This pull request adds a PopScope to StatefulNavigationShellState to handle back button navigation by switching to the root branch when the current branch cannot pop. A review comment identifies a potential issue where this logic could interfere with nested PopScope widgets and suggests checking the navigator's state before switching branches.

[gemini-code-assist]

The current implementation of PopScope might interfere with nested PopScope widgets within a branch.

If a child route on a non-root branch (e.g., Tab B) uses a PopScope to block a pop (for instance, to show a "Discard changes?" confirmation dialog), the didPop value in this shell-level PopScope will be false. Consequently, goBranch(0) will be triggered, switching the user to the root branch even though the intention was to stay on the current branch and handle the pop locally.

To fix this, you should check if the current branch's navigator can actually pop before deciding to switch branches. If navigator.canPop() is true but didPop is false, it implies a nested PopScope blocked the pop, and the shell should not intervene.

    return PopScope(
      canPop: widget.currentIndex == 0,
      onPopInvokedWithResult: (bool didPop, Object? result) {
        if (!didPop && widget.currentIndex != 0) {
          final NavigatorState? navigator =
              widget.shellRouteContext.navigatorKey.currentState;
          // Only switch to the root branch if the current branch navigator
          // cannot pop further. If it can pop but didn't, a nested PopScope
          // likely blocked it.
          if (navigator != null && !navigator.canPop()) {
            goBranch(0);
          }
        }
      },
      child: widget.containerBuilder(context, widget, children),
    );