Publish stable client proto inputs

src/MxGateway.Contracts/Protos/mxaccess_gateway.proto

@@ -30,6 +30,10 @@ message OpenSessionReply {
   repeated string capabilities = 5;
   google.protobuf.Duration default_command_timeout = 6;
   ProtocolStatus protocol_status = 7;
+  // Public gateway contract version implemented by this endpoint. Clients use
+  // this value to reject incompatible generated-code inputs before issuing
+  // command-specific MXAccess calls.
+  uint32 gateway_protocol_version = 8;
 }
 
 message CloseSessionRequest {