[segment cache]: fix interception route handling (#84497)
With `clientSegmentCache` enabled, if you are already on a route that
was intercepted/could have been intercepted, and then you navigated to
it again, the router would get stuck in a state where it was infinitely
suspended on data it would never receive.
When no prefetch is available, we assume that the FlightRouterState sent
by the server contains everything that is needed to describe the
rendering shape of the page. However this isn't quite true: in
`walk-tree-with-flight-router-state` (the function that populates
prefetch responses), we skip sending down the `__DEFAULT__` segment. As
a result, the client would get into a state where it thinks it needs to
request data from the server.
This updates the code to not special-case `__DEFAULT__` segments. We
should separately see how we can restore this optimization to keep those
payloads as small as possible, but it causes problems with back/forward
nav that need to be investigated.
