add test case for nextOperationId

Author: zhongkechen

sdk/src/test/java/com/amazonaws/lambda/durable/execution/ExecutionManagerTest.java

@@ -42,6 +42,15 @@ private Operation stepOp(String id, OperationStatus status) {
                 .build();
     }
 
+    @Test
+    void nextOperationId() {
+        var manager = createManager(List.of(executionOp()));
+        assertEquals("1", manager.nextOperationId());
+        assertEquals("2", manager.nextOperationId());
+        assertEquals("3", manager.nextOperationId());
+    }
+
+
     @Test
     void startsInReplayModeWhenOperationsExist() {
         var manager = createManager(List.of(executionOp(), stepOp("1", OperationStatus.SUCCEEDED)));