[go_router] Fix spurious onExit when refreshListenable re-parses a pushed route

[davidmigloz]

GoRoute.onExit fires spuriously for a pushed route that never left the navigation stack whenever refreshListenable fires (auth refresh, a tab-refocus listener) while that route is on top. If the route uses onExit for a "discard unsaved changes?" prompt, the user sees it on every refresh tick.

Root cause: ImperativeRouteMatch.completer is the ephemeral result channel for push(...).then(...) and can't survive a codec round-trip — a refreshListenable re-parse (and state restoration) decodes a brand-new Completer. The re-decoded match list therefore compares unequal to the live one even though nothing changed, defeating setNewRoutePath's no-op fast path; the router treats the re-parse as a real navigation change and runs onExit.

Fix: on the re-decode path only (the parser branch handling restoration, browser history, and refresh re-parses), carry over the live completer for each imperative match by pageKey (unique per push) via an internal RouteMatchList.reconcileImperativeCompleters. The re-decoded configuration then compares equal to the live one — completer included — so the fast path holds, onExit doesn't fire, and in-flight push futures are preserved. operator==/hashCode are untouched, and imperative navigations (push/replace/pushReplacement) never hit this path.

An earlier version of this PR excluded the completer from ImperativeRouteMatch equality instead. That regressed same-location replace() — whose match deliberately reuses pageKey and differs only by its new completer — orphaning the returned Future; see the PR thread for details.

Tests: no spurious onExit on a refresh tick; a live push future resolves across the tick; same-location replace() still completes its future; reconciliation contract tests in match_test.dart. Full go_router suite and analyzer green.

Fixes flutter/flutter#189578

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[gemini-code-assist]

Code Review

This pull request excludes the ephemeral completer from the ImperativeRouteMatch equality (==) and hashCode calculations to prevent spurious onExit calls when a route is re-decoded (e.g., during a refreshListenable tick). It also adds corresponding unit and widget tests to verify this behavior. One review comment suggests wrapping an unawaited router.push call in unawaited to satisfy the unawaited_futures lint rule.