fix navigator observer announcement order

[chunhtai]

Description

The navigator refactor accidentally changes the contract of navigator observer. If there are multiple route added in one history update (for example adding initial routes), the observer will receive the notifications from top-most route to bottm-most routes. It is the opposite of the order before the navigator observation.

This pr fixes it.

Related Issues

Fixes #57407 (comment)

Tests

I added the following tests:

See files

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[chunhtai]

@goderbauer This is somehow blocking the hero refactor since it touches the navigator observer

[goderbauer]

nit: maybe call this flushObserverNotifications? (similarly to the _flushRouteAnnouncement below).

[goderbauer]

As an idea: maybe use a polymorphic approach:

• remove this enum
• have an abstract void notify(List<Observers> observers) on _NavigatorObservation
• have subclasses for the various actions that implement the correct notification dispatch for the observation.

That also avoids the big switch below

[chunhtai]

ready for another look

[goderbauer]

LGTM