[go_router] Fixes "Bad state: Future already completed" when a route with onExit is popped twice

[iCodePoet]

When a route with an onExit callback is popped, GoRouterDelegate._handlePopPageWithRouteMatch vetoes the synchronous pop (returns false) and defers completion to a scheduleMicrotask. Because the page is still on the navigator until the configuration change is applied, a second back event arriving in that window (e.g. a user tapping the back button twice in quick succession) reaches the delegate with the same RouteMatchBase and schedules a second microtask. Both microtasks call _completeRouteMatch → ImperativeRouteMatch.complete() on the same completer, and the second call throws an unhandled StateError: Bad state: Future already completed.

This affects far more apps than explicit onExit users: any app using go_router_builder typed routes generated before the hasOverriddenOnExit parameter existed gets an implicit onExit wired by GoRouteData.$route (it treats a null hasOverriddenOnExit as true for backward compatibility), so every typed route takes the deferred pop path. We hit this in production from users double-tapping the AppBar back button.

This PR makes ImperativeRouteMatch.complete() ignore completions after the first, so the first pop's result is reported and the redundant second pop becomes a no-op (RouteMatchList.remove already returns the list unchanged when the match is gone). A regression test reproducing the double-pop is included; it fails with a StateError without the fix.

Fixes flutter/flutter#191280

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[gemini-code-assist]

Code Review

This pull request updates the go_router package to version 17.5.1. It addresses a StateError by ensuring that ImperativeRouteMatch.complete returns early if its underlying completer is already completed, preventing multiple completions when a route is popped twice before the navigator rebuilds. A corresponding regression test has been added to on_exit_test.dart, and the CHANGELOG.md has been updated. There are no review comments, so no additional feedback is provided.

[iCodePoet]

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