prefetchInlining: fix build crash on notFound() pages (#92806)
When traversing the route tree in collectPrefetchHints, we use keys from
the FlightRouterState to look up corresponding entries in the seed data
object. Usually these two structures agree, but there are subtle cases
like notFound() where the router state contains keys that don't exist in
the seed data. TypeScript doesn't catch this because the `in` operator
narrows the key to `string`, which is assignable to the seed data
object's index signature — but the lookup returns `undefined` rather
than `null`.
Eventually the plan is to converge these into a single data structure so
this kind of mismatch can't happen structurally. For now, coalesce
undefined to null with `?? null`, matching the pattern already used in
ppr-navigations.ts and segment-cache/cache.ts on the client.
## Test plan
- Added regression test: page with `notFound()` + `generateStaticParams`
under `prefetchInlining`
- Verified the build crashes without the fix and passes with it
- All existing prefetch-inlining tests continue to pass
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