go_router: clarify onEnter/redirect ordering, add regression test

[developerashkan]

What this does

Clarifies the onEnter doc comment on RoutingConfig to explicitly warn
that onEnter is evaluated once, against the literal navigated-to URI,
and is never re-invoked for locations reached only through a redirect:
chain. Adds a regression test locking in this behavior for the exact
scenario reported in flutter/flutter#188014, plus a second test
demonstrating the correct workaround (putting the guard inside
redirect: instead of onEnter).

Why

flutter/flutter#188014 reports what looks like a bug: an onEnter guard
checking for an intermediate redirect target (/a, reached via a
redirect: on /) never fires. Tracing the pipeline shows this is
actually intended, existing behavior — see the
'onEnter called once when top-level redirect chains' test (written for
the related #178984), which asserts the same ordering. The docs didn't
make this ordering/visibility gap explicit though, so it's an easy trap.
This PR closes that documentation gap and adds test coverage so the
behavior can't silently change without a test failing.

Related

Fixes flutter/flutter#188014 (as a documentation clarification, not a
behavior change — see discussion on the issue).

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[gemini-code-assist]

Code Review

This pull request updates the documentation for RoutingConfig.onEnter in go_router to clarify that onEnter is evaluated exactly once per navigation against the literal target URI, strictly before redirects run. It also adds regression tests in on_enter_test.dart to verify this behavior and demonstrate how to guard redirect targets. Feedback on the changes suggests simplifying a test route definition by removing an unused builder callback on a route that always redirects.

[developerashkan]

@googlebot I signed it!

[Piinks]

Hey @developerashkan thanks for contributing!

[stuartmorgan-g]

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