Derive foreground cache revalidation from the consumer (#96731)
## Summary
Foreground revalidation is necessary when a stale result will be
persisted by another server cache, not only when the request is
prerendering. Otherwise, a cache created during a dynamic request can
consume a stale inner value and extend its lifetime.
- derive foreground revalidation from whether the immediate work-unit
consumer will persist the result in a server cache
- model that capability as willConsumerServerCache rather than
inheriting it through the full outer scope chain
- treat server cache and static prerender work units as server-caching
consumers, while runtime prerenders are not
- add regression coverage for an outer "use cache" scope consuming a
stale unstable_cache entry
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