Fix stopping issue when forecaster is in FORECAST_FAILURE state (#1502)

Mark FORECAST_FAILURE as a non-ending state so TaskManager recognizes the task as stoppable. Previously, TaskManager.stopLatestRealtimeTask failed to stop the task, as `isDone()` returned false for FORECAST_FAILURE, causing a "job is already stopped" error.

Testing:
- Manually verified forecaster can now be stopped successfully from FORECAST_FAILURE state.

Signed-off-by: Kaituo Li <kaituo@amazon.com>

Author: kaituo

release-notes/opensearch-anomaly-detection.release-notes-3.1.0.0.md

@@ -10,4 +10,5 @@ Compatible with OpenSearch 3.1.0
 - Fix incorrect task state handling in ForecastRunOnceTransportAction ([#1489](https://github.com/opensearch-project/anomaly-detection/pull/1489))
 - Fix incorrect task state handling in ForecastRunOnceTransportAction ([#1493](https://github.com/opensearch-project/anomaly-detection/pull/1493))
 - Refine cold-start, window delay, and task updates ([#1496](https://github.com/opensearch-project/anomaly-detection/pull/1496))
+- Fix stopping issue when forecaster is in FORECAST_FAILURE state ([#1502](https://github.com/opensearch-project/anomaly-detection/pull/1502))
 

src/main/java/org/opensearch/timeseries/model/TaskState.java

@@ -90,7 +90,8 @@ public String getDescription() {
             TaskState.RUNNING.name(),
             INIT_TEST.name(),
             AWAITING_DATA_TO_INIT.name(),
-            AWAITING_DATA_TO_RESTART.name()
+            AWAITING_DATA_TO_RESTART.name(),
+            FORECAST_FAILURE.name()
         );
 
     public static boolean isAwaitState(String state) {

src/test/java/org/opensearch/timeseries/model/TaskStateTests.java

@@ -31,13 +31,14 @@ public void testNotEndedStatesConstant() {
         List<String> list = TaskState.NOT_ENDED_STATES;
 
         // expected size and membership
-        assertEquals(6, list.size());
+        assertEquals(7, list.size());
         assertTrue(list.contains(TaskState.CREATED.name()));
         assertTrue(list.contains(TaskState.INIT.name()));
         assertTrue(list.contains(TaskState.RUNNING.name()));
         assertTrue(list.contains(TaskState.INIT_TEST.name()));
         assertTrue(list.contains(TaskState.AWAITING_DATA_TO_INIT.name()));
         assertTrue(list.contains(TaskState.AWAITING_DATA_TO_RESTART.name()));
+        assertTrue(list.contains(TaskState.FORECAST_FAILURE.name()));
 
         // an obviously ended state must be absent
         assertFalse(list.contains(TaskState.FINISHED.name()));