Fix proto parsing.
Fix proto parsing.

Unfortunately `MergePartialFromBoundedZeroCopyStream(_, -1)` does not work
(while `MergeFromImpl(BoundedZCIS)` interprets -1 as no limit, at the end it
checks `ctx.EndedAtLimit()` rather than `ctx.EndedAtEndOfStream()` and thus
it fails).

Hence we use `ParsePartialFromZeroCopyStream()`, which is possible only when
`!options.merge()`. Otherwise we defer to the most general and most expensive
`CodedInputStream`.

PiperOrigin-RevId: 453865556

google/riegeli,google/riegeli,google/riegeli,google/riegeli