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).
Fail the oldest in-flight request for the given reducer name. Used when
the server returns a protocol-level failure (e.g. "no such reducer")
with requestId=0 because it cannot associate the failure with a
specific outbound call. Without this fallback, such failures silently
miss the requestId lookup in completeRequest and the pending request
sits until _timeoutRequest fires 10s later.