fix: Flush outputStream after write (#22660)

transfer() method should call flush() on the OutputStream
before returning to ensure all buffered data is actually written

Author: caalador

flow-server/src/main/java/com/vaadin/flow/server/streams/TransferUtil.java

@@ -116,6 +116,7 @@ public static long transfer(InputStream inputStream,
                 }
             }
         }
+        outputStream.flush();
         long finalTransferred = transferred;
         listeners.forEach(listener -> listener.onComplete(transferContext,
                 finalTransferred));