[Bug] [connector-doris] DorisStreamLoad loading state mismanagement causes RecordBuffer infinite loop during shutdown

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/seatunnel/issues?q=is%3Aissue+label%3A%22bug%22) and found no similar issues.

### What happened
In the exception scenario where the doris fe server returns a status code of 200(for example, an exception like AnalysisException occurs on the fe server)), the following problems will occur

1. In` DorisStreamLoad.stopLoad()`, the loading state is set to false prematurely, even before the `httpClient.execute` method returns a response result. This improper timing prevents subsequent error detection mechanisms that rely on the loading state from functioning correctly.
2. This prevents `getLoadFailedMsg() `from properly collecting error msg form http response
3. The loop in RecordBuffer.stopBufferData() may wait indefinitely due to inability to retrieve error messages
https://github.com/apache/seatunnel/blob/40320d866afe51da223dee2da33a5e7c9faecddc/seatunnel-connectors-v2/connector-doris/src/main/java/org/apache/seatunnel/connectors/doris/sink/writer/RecordBuffer.java#L87-L90
5. This ultimately leads to the task being trapped in a dead loop/infinite loop and causing the task to hang/terminate abnormally

There is also another problem in DorisSinkWriter
https://github.com/apache/seatunnel/blob/40320d866afe51da223dee2da33a5e7c9faecddc/seatunnel-connectors-v2/connector-doris/src/main/java/org/apache/seatunnel/connectors/doris/sink/writer/DorisSinkWriter.java#L256-L267
1. Incomplete resource cleanup: When flush() throws an IOException, subsequent resource cleanup steps are skipped
2. Thread pool not closed: scheduledExecutorService can not be closed, causing background threads to continue running
3. Connection not released: dorisStreamLoad may not be properly closed, maintaining the connection with Doris

### SeaTunnel Version

dev

### SeaTunnel Config

```conf
Created from the web, without config
```

### Running Command

```shell
Created from the web, without command
```

### Error Exception

```log
Doris task blocked by infinite loop and unable to terminate
```

### Zeta or Flink or Spark Version

_No response_

### Java or Scala Version

_No response_

### Screenshots

The task cannot be terminated normally

### Are you willing to submit PR?

- [x] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)