bugfix: propagate staleTime to seeded prefetch entry (#81263)
In #71280, we hooked up the server-side `staleTime` header to the
client-side prefetch cache. This means that if the server responds with
a staleTime value, the client router will use that rather than the
previous heuristic when determining if a prefetch entry can be reused.
However, there was missing functionality to set a proper staleTime value
for the initially seeded prefetch entry (aka the first page visit). In
the case of customizing staleTimes, this meant that navigations back to
the original page would defer back to the old stale heuristic, which
would differ from subsequent navigations that were properly seeded with
a staleTime.
Separately, I believe that it's problematic that if the `staleTime`
header is present, that we honor that and only set a fresh/stale cache
status, because the newer heuristic doesn't consider the `reusable`
cache status. This should probably be flagged to the clientSegmentCache
flag. However, to minimize the changes here, I've only addressed this
particular case.
