Old logic from the pages router was previously being hit during
development. This was more apparent when PPR was enabled as it was
mixing dynamic + static rendering in development which propagated to
errors. This change ensures that requests that are made with `RSC: 1`
are not marked as `/_next/data` URL's, and don't use the same logic
paths.
Previously it was a bit confusing because we used the variable
`isDataReq` in a few places that made it hard to tell what it was
referring to. In this case, the `isDataReq` is actually only used by the
pages router. This renames the `isDataReq` to `isNextDataRequest` to
make it clearer, as well as refactors to ensure that it's not used in
the paths for app routes.
Also to better represent the rendering modes the `supportsDynamicHTML`
variable was renamed to `supportsDynamicResponse`.
Fixes#66241
b5d911c92c