fix race condition on OrchestrationRuntimeState.NewEvents (#380)

* fix race condition on mutable list of new events inside OrchestrationRuntimeState.

* Apply suggestions from code review

Co-authored-by: David Justo <david.justo.1996@gmail.com>

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Co-authored-by: David Justo <david.justo.1996@gmail.com>

src/DurableTask.Netherite/OrchestrationService/NetheriteOrchestrationService.cs

@@ -876,7 +876,7 @@ namespace DurableTask.Netherite
                 InstanceId = workItem.InstanceId,
                 BatchStartPosition = messageBatch.BatchStartPosition,
                 BatchLength = messageBatch.BatchLength,
-                NewEvents = (List<HistoryEvent>)newOrchestrationRuntimeState.NewEvents,
+                NewEvents = newOrchestrationRuntimeState.NewEvents.ToList(), // `NewEvents` in `newOrchestrationRuntimeState` may be mutated, so we copy to avoid a surprise change.
                 WorkItemForReuse = cacheWorkItemForReuse ? orchestrationWorkItem : null,
                 PackPartitionTaskMessages = partition.Settings.PackPartitionTaskMessages,
                 PersistFirst = partition.Settings.PersistStepsFirst ? BatchProcessed.PersistFirstStatus.Required : BatchProcessed.PersistFirstStatus.NotRequired,