[go_router] Fix pop() restoring stale configuration when route has onExit

[dkajiwara]

GoRouter.pop() unconditionally called restore() after routerDelegate.pop(). When the route has onExit, the pop is deferred to a microtask, so restore() was called with the pre-pop configuration. This caused async redirects to overwrite
the post-pop state, making the popped route reappear.

Fixed by comparing currentConfiguration identity before and after pop. restore() is only called when the configuration was actually updated (synchronous pop).

Fixes flutter/flutter#174525 flutter/flutter#180002

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[gemini-code-assist]

Code Review

This pull request fixes an issue where GoRouter.pop() could restore a stale configuration when a route has an onExit handler, which could lead to the popped route reappearing. The fix cleverly uses an identical() check to ensure restore() is only called for synchronous pops where the configuration has actually changed. The changes are well-supported by new regression tests that cover the fixed scenario and related edge cases. My review includes one suggestion to enhance a code comment for better long-term maintainability.

[gemini-code-assist]

The current comment explains the condition well. To further improve maintainability, consider adding a brief note about why this check is important, mentioning the risk of restoring a stale configuration. This will help future developers understand the full context and prevent accidental regressions.

// Only restore on a synchronous pop to avoid restoring a stale configuration.
// A deferred pop (e.g. due to an async `onExit`) would leave the
// `currentConfiguration` instance unchanged, and restoring it could cause issues.