[ML] Mark task as completed when DFA job is stopped while reindexing

After elastic#54650 we catch `TaskCancelledException` when we wait for
reindexing to complete as it may be thrown. However, when that happens
we do not mark the task as completed. This results in the stop request
never returning and the failures we saw in elastic#55068.

Closes elastic#55068

x-pack/plugin/ml/qa/native-multi-node-tests/src/test/java/org/elasticsearch/xpack/ml/integration/RunDataFrameAnalyticsIT.java

@@ -591,7 +591,6 @@ public void testLazyAssignmentWithModelMemoryLimitTooHighForAssignment() throws
             "Stopped analytics");
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/55068")
     public void testOutlierDetectionStopAndRestart() throws Exception {
         String sourceIndex = "test-outlier-detection-stop-and-restart";
 

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/dataframe/DataFrameAnalyticsManager.java

@@ -228,6 +228,7 @@ private void reindexDataframeAndStartAnalysis(DataFrameAnalyticsTask task, DataF
                 if (error instanceof TaskCancelledException && task.isStopping()) {
                     LOGGER.debug(new ParameterizedMessage("[{}] Caught task cancelled exception while task is stopping",
                         config.getId()), error);
+                    task.markAsCompleted();
                 } else {
                     task.setFailed(ExceptionsHelper.unwrapCause(error).getMessage());
                 }