skip seeding prefetch cache in development (#72191)
Since prefetching is disabled in development, seeding the prefetch cache
for the initially rendered page can lead to an inconsistent navigation
experience, where the initially visited page won't behave the same as
subsequent pages that you navigate to.
We should ultimately get to a place where the prefetch behavior is
consistent between dev/start to keep the production behavior as
consistent as possible with the development experience, but when we do
so, we would want to enable it across the board.
This happens to fix a bug with dynamicIO because the server-patch action
(which happens when data is missing for a rendered segment) was
mismatching the router state tree, which triggers a hard navigation to
recover. This happens to fix the issue because the router never hits the
server patch case, which is when the hard navigation could occur.
Separately, we're working to verify why the seeded prefetch entry might
have caused this change in behavior only in dev.
Note: this modifies a navigation test that was asserting on RSC requests
taking place, which will now happen in dev as there'll be no prefetch
entry.
Fixes #72150
