fix: ensure batch values are sharable (#58315)
Previously, the incremental cache returned objects with a readable stream (that were consumed). These can't be shared. Instead, this modifies the cache to instead return strings which are sharable.
This also fixes a bug related to revalidation when writing an updated prefetch RSC payload to the filesystem when PPR is enabled. Previously it just wrote to `.rsc` and now it correctly writes a `.prefetch.rsc` file instead.
