I have a question about DEFAULT_START_MESSAGE_OFFSET

`public static final long DEFAULT_START_MESSAGE_OFFSET = -1;`

Why is the default value -1? It causes the following error：

```
java.lang.RuntimeException: org.apache.rocketmq.client.exception.MQClientException: offset < 0
For more information, please visit the url, http://rocketmq.apache.org/docs/faq/
at org.apache.rocketmq.flink.legacy.RocketMQSourceFunction.lambda$null$1(RocketMQSourceFunction.java:339) ~[blob_p-b39848a6520d4473030e80a1af68842bda0d1497-439037d8a6ae48b5fa69464d73041c8f:?]
at org.apache.rocketmq.flink.legacy.common.util.RetryUtil.call(RetryUtil.java:46) ~[blob_p-b39848a6520d4473030e80a1af68842bda0d1497-439037d8a6ae48b5fa69464d73041c8f:?]
at org.apache.rocketmq.flink.legacy.RocketMQSourceFunction.lambda$run$2(RocketMQSourceFunction.java:243) ~[blob_p-b39848a6520d4473030e80a1af68842bda0d1497-439037d8a6ae48b5fa69464d73041c8f:?]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_302]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_302]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_302]

```

Start by locating DEFAULT_START_MESSAGE_OFFSET and tracing the offset handling through RocketMQSourceFunction.java:243 and :339, then inspect RetryUtil.java:46. Determine why -1 reaches the client and whether the issue calls for a documented explanation or a behavioral change; done requires an agreed resolution that avoids the reported invalid-offset error.

Written by the indexing model from the issue text.