Completes the pending request with the given requestId. Returns true
if the request was found and completed, false if no pending request
matched (e.g. requestId=0 from a protocol-level server error).
BSATN-encoded args the SDK sent for the in-flight request. Used by the
caller-side ReducerResult handler to construct the ReducerEvent
without re-decoding from the (non-existent) wire copy.