Fix prefetch hints not being loaded during revalidation (#92625)
The prefetch hints manifest was not loaded during revalidation renders,
causing all segments to fall back to PrefetchDisabled — effectively
disabling prefetching for that route until the next full build.
The app-page template handler constructs its own renderOpts from scratch
without including prefetch-hints.json. Normal requests go through the
server's renderOpts (which loads the manifest at startup), but
background revalidation renders go through the template handler, so the
hints were always empty.
Load the manifest in the route module's loadManifests() and pass it
through to renderOpts.
Closes #92255
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Co-authored-by: Zack Tanner <1939140+ztanner@users.noreply.github.com>