add presuspend sanity check

Author: zhongkechen

sdk/src/main/java/software/amazon/lambda/durable/execution/ExecutionManager.java

@@ -223,11 +223,26 @@ public void deregisterActiveThread(String threadId) {
 
             if (activeThreads.isEmpty()) {
                 logger.info("No active threads remaining - suspending execution");
+                preSuspendCheck();
                 suspendExecution();
             }
         }
     }
 
+    private void preSuspendCheck() {
+        if (executionExceptionFuture.isDone()) {
+            throw new IllegalStateException("Execution already suspended");
+        }
+
+        operationStorage.values().stream()
+                .filter(op -> !isTerminalStatus(op.status()))
+                .findFirst()
+                .ifPresentOrElse(op -> logger.debug("Some operations pending"), () -> {
+                    logger.warn("There is no operation still in progress");
+                    throw new IllegalStateException("Cannot suspend execution without an operation in progress");
+                });
+    }
+
     // ===== Checkpointing =====
 
     // This method will checkpoint the operation updates to the durable backend and return a future which completes