Generate per-segment responses for any static page (#73945)
Originally I gated per-segment prefetch generation on the PPR flag,
because I thought the client Segment Cache would require PPR to be
enabled on the server. However, since then the strategy has evolved and
I do think we can roll out the Segment Cache independently of PPR.
Dynamic pages without PPR won't be able to take full advantage of the
Segment Cache, but if the page is fully static then there's no reason we
can't implement all the same behavior.
So during per-segment prerendering, I've changed the feature condition
to check for the `clientSegmentCache` flag instead of the PPR one.
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Additionally, when I broadened the feature check, a failing test
revealed that I neglected to set up error digest decoding correctly, so
the first commit in this stack fixes that.
