[Enhancement] In RocketMQ 5.3.4, the rejection policy of the thread pool is set redundantly in the init method of the GrpcMessagingApplication class

### Before Creating the Enhancement Request

- [x] I have confirmed that this should be classified as an enhancement rather than a bug/feature.

### Summary

In RocketMQ 5.3.4, the rejection policy of the thread pool is set redundantly in the init method of the GrpcMessagingApplication class，the code is as follows :
protected void init() {
GrpcTaskRejectedExecutionHandler rejectedExecutionHandler = new GrpcTaskRejectedExecutionHandler();
this.routeThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler);
this.routeThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler);
this.producerThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler);
this.consumerThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler);
this.clientManagerThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler);
this.transactionThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler);
}

### Motivation

this.routeThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler)执行了两次，没有必要。

### Describe the Solution You'd Like

the improved code is as follows :
protected void init() {
GrpcTaskRejectedExecutionHandler rejectedExecutionHandler = new GrpcTaskRejectedExecutionHandler();
this.routeThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler);
this.producerThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler);
this.consumerThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler);
this.clientManagerThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler);
this.transactionThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler);
}

### Describe Alternatives You've Considered

the improved code is as follows :
protected void init() {
GrpcTaskRejectedExecutionHandler rejectedExecutionHandler = new GrpcTaskRejectedExecutionHandler();
this.routeThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler);
this.producerThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler);
this.consumerThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler);
this.clientManagerThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler);
this.transactionThreadPoolExecutor.setRejectedExecutionHandler(rejectedExecutionHandler);
}

### Additional Context

_No response_

Open the GrpcMessagingApplication class and inspect its protected init method, focusing on the repeated routeThreadPoolExecutor rejection-handler assignment. Remove the redundant assignment and confirm that each listed executor is configured once.

Written by the indexing model from the issue text.