[go_router] Fix key collisions when sibling ShellRoutes coexist during push - #11143

…#12549)

This supersedes #11143, which was closed as inactive. It incorporates the review feedback from that PR.

## Problem

When `push()` navigates between sibling or nested regular `ShellRoute` branches, multiple shell pages and navigators can coexist in the widget tree.

This could produce duplicate page or widget keys because:

- `_CustomNavigator` was keyed using `GlobalObjectKey(navigatorKey.hashCode)`, so distinct navigator keys with the same hash were treated as the same key.
- `ShellRouteMatch.pageKey` was derived only from `route.hashCode`.
- Re-entering a `ShellRoute` that was already present in the imperative stack reused its page and navigator identities.

The first implementation in #11143 created fresh page and navigator keys for every `ShellRouteMatch`. Review correctly pointed out that this made navigator identity unstable across route reparses and prevented the configured `ShellRoute.navigatorKey` from consistently identifying its navigator.

`RoutingConfig` updates could also leave inactive loaded `StatefulShellRoute` branches holding matches and navigator widgets built from the old route objects.

## Fix

This version addresses that feedback by:

- preserving the configured navigator key during normal route matching and reparsing;
- making shell page keys identity-aware, so distinct routes remain distinct even when their `hashCode` values collide;
- keying the nested `_CustomNavigator` wrapper with a value-equal `ValueKey` of its navigator key;
- assigning scoped page and navigator keys only when an imperative push would otherwise collide with another live regular `ShellRoute` match;
- deriving that scope from the persistent `ImperativeRouteMatch.pageKey`;
- carrying existing shell matches through recursive imperative push matching, so nested key collisions are detected;
- reparsing loaded inactive `StatefulShellRoute` branches and rebuilding their cached navigator widgets when route objects change;
- scoping saved matches to their shell and resetting branches whose base location was removed;
- identifying reparsed branches by their navigator key, including after nested `pushReplacement()`;
- preserving branch and root observers when rebuilding inactive or preloaded navigators, honoring `notifyRootObserver`;
- leaving the root navigator wrapper and normal configured-key navigation unchanged.

## Tests

The regression test `push to a sibling shell route under the same parent shell route` forces both navigator-key and route-hash collisions.

It covers:

- pushing to a sibling shell route;
- re-entering the first shell route while its original instance is still in the stack;
- preserving navigator and page state across match-list reconstruction and `refresh()`;
- detecting and scoping collisions in nested `ShellRouteMatch` trees;
- popping back through both shell routes;
- navigating with `go()` after the imperative stack is removed.

Additional regressions cover the nested push sequence in [the review comment](#12549 (comment)), inactive loaded branches after a `RoutingConfig` update ([review comment](#12549 (comment))), and the exact flows from flutter/flutter#148712 and flutter/flutter#140586.

Eight additional regression cases cover the follow-up review findings: a removed base route with a surviving push (with and without preload), nested `pushReplacement()`, a removed pushed route, and branch/root observer behavior for both preload and `notifyRootObserver` settings. The observer regression uses pageless `Navigator.push()` so a GoRouter rebuild cannot mask detached observers; it also verifies navigator-state preservation and updated `clipBehavior`.

No screenshots are included because this is a routing/runtime fix.

- Fixes flutter/flutter#148712
- Fixes flutter/flutter#140586

## Validation

- `flutter test --no-pub`: 471 passed
- `flutter analyze --no-pub`: no issues
- `flutter_plugin_tools validate --check-for-missing-changes --packages go_router`: no issues
- `flutter_plugin_tools publish-check --packages go_router`: no issues
- License check: passed

Release note: This package uses batch releases, so the fix is recorded in `pending_changelogs/shell_route_key_collisions.yaml`; `pubspec.yaml` and `CHANGELOG.md` remain unchanged.

## Pre-Review Checklist

[^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.