[partial fallbacks]: enable by default (#92627)
This makes `experimental.partialFallbacks` the default when
`cacheComponents` is enabled.
For routes that prerender only a subset of params via
`generateStaticParams`, requests for unknown params used to either
suspend to a nearby fallback or block until the full route finished
rendering. In the cases where the route could already produce a
contentful shell because param access happened below a Suspense
boundary, the old default still served that generic shell on every
request.
With `partialFallbacks` enabled by default, the first request still
receives the best shell available for that route, but Next.js now
revalidates in the background using the newly discovered param values.
That allows subsequent requests to reuse a more specific cached result:
- If the route becomes fully static for those params, later requests get
the fully prerendered page.
- If the leaf remains dynamic, Next.js upgrades the shell as far as it
can, filling in the newly known static params while the dynamic portion
continues to stream.
This also fixes a bug that was caught now that we ran this flag against
the entire CI. A concrete path like `/optional-catchall` could match the
generic `/optional-catchall/[[...slug]]` shell and incorrectly serve
fallback-shell behavior even though that exact path was already
prerendered. This change keeps concrete prerendered paths serving their
concrete result instead of downgrading back to the generic shell.
This PR also removes redundant `partialFallbacks: true` test config and
adds coverage for the explicit opt-out behavior (`partialFallbacks:
false`).
