data pipeline refactor and fix

protobuf/ohlc.proto

@@ -58,4 +58,8 @@ message OHLCBatchMetadata {
 
   // Error message if status is ERROR
   optional string error_message = 8;
+
+  // True on the final page of a historical query (including error/not-found markers).
+  // Flink publishes HistoryReadyNotification only when this is true.
+  bool is_last_page = 9;
 }