Fix `isResponsePartial` for non-PPRRuntime prefetches
The `isResponsePartial` value from `cacheData` (set by `processFetch`)
defaults to `true` when no marker byte is found. This is correct for
navigation requests (conservative default), but wrong for Full and
LoadingBoundary prefetches which are by definition complete.
Before the `processFetch` refactoring, the prefetch path only read
`isResponsePartial` from `stripIsPartialByte` for PPRRuntime, and
defaulted to `false` for other strategies. The refactoring changed it to
always read from `cacheData`, which overrode the correct `false` default
with an incorrect `true` for non-PPRRuntime strategies.
Fix: only use `cacheData.isResponsePartial` for PPRRuntime prefetches;
default to `false` for Full and LoadingBoundary.
