[FLINK-39388][tests] Fix flaky DataGeneratorSourceITCase#testGatedRateLimiter

- Add !firstCheckpointCompleted guard in notifyCheckpointComplete to
  prevent redundant state updates after the first checkpoint has
  already been marked complete

Author: featzhang

flink-connectors/flink-connector-datagen-test/src/test/java/org/apache/flink/connector/datagen/source/DataGeneratorSourceITCase.java

@@ -288,7 +288,9 @@ public void snapshotState(FunctionSnapshotContext context) throws Exception {
         @Override
         public void notifyCheckpointComplete(long checkpointId) throws Exception {
             // Stop collecting elements once the first checkpoint has completed.
-            if (checkpointId >= firstCheckpointId && firstCheckpointId != Long.MIN_VALUE) {
+            if (!firstCheckpointCompleted
+                    && checkpointId >= firstCheckpointId
+                    && firstCheckpointId != Long.MIN_VALUE) {
                 firstCheckpointCompleted = true;
             }
         }