Fix `ZeroCopyCodedInputStream::ReadCord()`

`ZeroCopyCodedInputStream::ReadCord()` is implemented in terms of `CodedInputStream::ReadCord()` which is incorrect: the contract for the interface method `ZeroCopyInputStream::ReadCord()` is that it appends to the provided cord output where `CodedInputStream::ReadCord()` sets the cord output value, clearing any pre-existing contents. This change fixes the internal implementation to append to any pre-existing contents.

PiperOrigin-RevId: 500872467