Refactor 'excludeJobIds' Serialization for Spark Job Compatibility

This commit changes the serialization format of the 'excludeJobIds' field in the session document. Previously serialized as an array (e.g., ["id1", "id2"]), this field is now formatted as a comma-separated string ("id1,id2"). This adjustment is made to ensure compatibility with plugins that currently do not support array parsing. This simplification aids in seamless integration and easier data handling in downstream processes.

Testing done:
* modified unit test
Signed-off-by: Kaituo Li <[email protected]>

flint-spark-integration/src/main/scala/org/opensearch/flint/app/FlintInstance.scala

@@ -120,7 +120,12 @@ object FlintInstance {
         "state" -> job.state,
         // update last update time
         "lastUpdateTime" -> currentTime,
-        "excludeJobIds" -> job.excludedJobIds,
+        // Convert a Seq[String] into a comma-separated string, such as "id1,id2".
+        // This approach is chosen over serializing to an array format (e.g., ["id1", "id2"])
+        // because it simplifies client-side processing. With a comma-separated string,
+        // clients can easily ignore this field if it's not in use, avoiding the need
+        // for array parsing logic. This makes the serialized data more straightforward to handle.
+        "excludeJobIds" -> job.excludedJobIds.mkString(","),
         "jobStartTime" -> job.jobStartTime))
   }
 }

flint-spark-integration/src/test/scala/org/opensearch/flint/app/FlintInstanceTest.scala

@@ -46,7 +46,9 @@ class FlintInstanceTest extends SparkFunSuite with Matchers {
     json should include(""""sessionId":"session-789"""")
     json should include(""""state":"RUNNING"""")
     json should include(s""""lastUpdateTime":$currentTime""")
-    json should include(""""excludeJobIds":["job-101","job-202"]""")
+    json should include(
+      """"excludeJobIds":"job-101,job-202""""
+    ) // Check for comma-separated string
     json should include(""""jobStartTime":1620000001000""")
     json should include(""""error":""""")
   }