[go_router] Add regression test for synchronous onEnter Block.then(router.go) (flutter#179370)

[NadeemIqbal]

Adds a regression test for flutter/flutter#179370. The user-visible symptom — a synchronous `onEnter` returning `Block.then(() => router.go(...))` failing to redirect on the initial route — was fixed in go_router 17.2.0 by #11136 (which replaced `await Future.sync(callback)` with `scheduleMicrotask(() async { await callback(); })` in `_OnEnterHandler.handleTopOnEnter` to break the re-entrant parse cycle), but the linked issue is still open and there is no test that fails when that change is reverted.

The new test in `packages/go_router/test/on_enter_test.dart` mirrors the exact predicate the reporter used (`current.uri.path == '/dashboard'` on the initial parse, where `currentState` equals `nextState` because the router has no committed configuration yet) and asserts that the sync `Block.then(router.go('/login'))` redirect resolves to `/login`.

I verified the test is a real regression test by temporarily reverting parser.dart to the pre-#11136 implementation (`await Future.sync(callback)`) and confirming this test fails with that change in place; with the current implementation it passes alongside the rest of `on_enter_test.dart` (32/32, all locally with stable Flutter SDK 3.41.7).

Fixes flutter/flutter#179370.

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. `[shared_preferences]`
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under.
• I updated/added any relevant documentation (doc comments with `///`).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under.
• All existing and new tests are passing.

Test-only change, so no version bump / CHANGELOG entry per the published-package exemption. Happy to bump if the reviewer prefers.

[gemini-code-assist]

Code Review

This pull request adds a regression test for issue #179370 in packages/go_router/test/on_enter_test.dart to verify that synchronous onEnter redirects on the initial route function correctly. The reviewer suggested removing the redundant localRouter variable and using the group-level router variable directly to maintain consistency with the existing test suite.

[gemini-code-assist]

The localRouter variable is redundant. You can assign the GoRouter instance directly to the group-level router variable, which is consistent with the implementation pattern used in other tests within this file.

Suggested change

	late GoRouter localRouter;
	localRouter = GoRouter(
	router = GoRouter(

[gemini-code-assist]

Update these lines to use the group-level router variable directly and remove the redundant localRouter assignment to maintain consistency with the rest of the test suite.

Suggested change

	router = localRouter;
	await tester.pumpWidget(MaterialApp.router(routerConfig: localRouter));
	await tester.pumpAndSettle();
	expect(localRouter.state.uri.path, equals('/login'));
	await tester.pumpWidget(MaterialApp.router(routerConfig: router));
	await tester.pumpAndSettle();
	expect(router.state.uri.path, equals('/login'));