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

[Gorniv]

This PR fixes key collisions in go_router when push() navigates from one ShellRoute branch to a sibling ShellRoute branch under the same parent.

In that scenario, both shell branches can temporarily coexist in the widget tree. Before this change, that could cause framework assertions such as Multiple widgets used the same GlobalKey because:

• _CustomNavigator was keyed with GlobalObjectKey(navigatorKey.hashCode), which could collide when two navigator keys had the same hashCode
• ShellRouteMatch reused the configuration-time ShellRoute.navigatorKey
• ShellRouteMatch.pageKey was derived only from route.hashCode, so multiple matches for the same ShellRoute could share page identity

This PR fixes the problem for regular ShellRoute by:

• creating a fresh GlobalKey<NavigatorState> for each ShellRouteMatch
• deriving a unique pageKey from that per-match navigator key
• keying _CustomNavigator with GlobalObjectKey(navigatorKey) instead of GlobalObjectKey(navigatorKey.hashCode)

The change is intentionally scoped to ShellRoute only. StatefulShellRoute keeps its existing stable pageKey behavior because applying per-match page identity there can create duplicate StatefulNavigationShell instances with the same internal GlobalKey.

This PR also adds a regression test that reproduces the old crash by forcing a navigator key hash collision, verifies that pushing to a sibling shell route no longer crashes, and confirms that the previous shell route state is preserved after pop().

No screenshots are included since this is a routing/runtime fix rather than a visual UI change.

• 'Multiple widgets used the same GlobalKey' on Hot Reload When Using ShellRoute with Global Key and Builder Parameter flutter#148712
• [go_router] Failed assertion: line 3817 pos 18: '!keyReservation.contains(key)': is not true. flutter#140586

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[gemini-code-assist]

Code Review

This pull request resolves key collision issues within go_router that occurred when navigating between sibling ShellRoute branches. The fix involves generating unique navigator and page keys for each ShellRouteMatch and using the navigator key object directly for _CustomNavigator's key instead of its hash code. These changes are well-tested with a new regression test that forces key hash collisions to verify the fix's robustness. The implementation is sound and effectively addresses the described problem.