Wire doesn’t support RPC methods with `StringValue` return types, or any wrappers

import "google/protobuf/wrappers.proto";

service StringValueService {
  rpc Hello(google.protobuf.StringValue) returns (google.protobuf.StringValue) {}
}

Wire turns StringValue into String?, and that doesn’t work because GrpcCall requires response and request types are non-null.

I made a quick attempt at supporting this, but I ran into a brick wall with MessageSource.read(). That function uses null to indicate end-of-stream, which collides with the potential for a true null response value.

Use the shown StringValueService definition as the reproduction case. Read MessageSource.read() and the GrpcCall request and response constraints first, then trace how Wire turns wrapper types into nullable values. Done means wrapper-based RPC methods can be supported without confusing a true null response with end-of-stream.