Refactor navigateReducer to handle mutable consistently (#45555
Creates a separate handleMutable function that ensures the initial call
and the second call are the same.
This only changes navigateReducer right now but I'll reuse the same
handling for serverPatch and refresh too when this lands.
Additionally found a bug where mpaNavigation was checked incorrectly
with `forceOptimisticNavigation`.
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## Bug
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