[ISSUE #6316]Remove unnecessary .as_str() call in DefaultMQProducer#start (#6318)

- Replaced .clone().as_str() with cleaner approach using intermediate variable
- CheetahString clone is still needed due to borrow checker requirements
- Leverages automatic coercion from &CheetahString to &str via AsRef/Deref
- Makes code more idiomatic and readable
- All tests passing

Author: voidcommit-afk

rocketmq-client/src/producer/default_mq_producer.rs

@@ -692,7 +692,8 @@ impl DefaultMQProducer {
 
 impl MQProducer for DefaultMQProducer {
     async fn start(&mut self) -> rocketmq_error::RocketMQResult<()> {
-        let producer_group = self.with_namespace(self.producer_config.producer_group.clone().as_str());
+        let producer_group_clone = self.producer_config.producer_group.clone();
+        let producer_group = self.with_namespace(&producer_group_clone);
         self.set_producer_group(producer_group);
         let default_mqproducer_impl = self
             .default_mqproducer_impl