Restore partial fallback shell upgrade coverage (#96096)
### Why?
Deployment tests for `partial-fallback-shell-upgrade` repeatedly fail
after #96074 because the fixture still expects partial fallback shell
upgrades without opting into Partial Prefetching. #96074 intentionally
emits `partialFallback` metadata only when `partialPrefetching` is
enabled.
Failed deployment test:
https://github.com/vercel/next.js/actions/runs/30008120037/job/89223758358#step:32:464
Passed:
https://github.com/vercel/next.js/actions/runs/30031537314/job/89297370591#step:32:107
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