optimize: expand unit test coverage for the [rocketmq] module.

[psxjoy]

• I have registered the PR changes.
• I have optimized the code using seata-codestyle.xml.
• I have checked all codes using seata-checkstyle.xml.

Ⅰ. Describe what this PR did

Add unit test cases to complete the unit testing objectives for the seata-rocketmq module.
增加单元测试用例，完成 rocketmq模块的单元测试目标

	Instructions Cov.	Branches Cov.	Missed	Cxty	Missed	Lines	Missed	Methods	Missed	Classes
Before	31%	8%	27	40	67	98	10	22	0	4
After	95%	83%	8	41	5	101	2	23	0	4

Ⅱ. Does this pull request fix one issue?

fix #6506

Ⅲ. Why don't you add test cases (unit test/integration test)?

Not applicate
不适用

Ⅳ. Describe how to verify it

Run mvn clean test

Ⅴ. Special notes for reviews

In the SeataMQProducer class, there's a method doSendMessageInTransaction, which calls the parent class method send(Message msg, long timeout). Since Mockito can't mock certain parent methods, the functionality has been refactored to ensure the unit test coverage exceeds 70%. The changes are as follows:

SeataMQProducer类中有个函数doSendMessageInTransaction，里面调用了父类的函数 send(Message msg, long timeout)。
因为Mockit 无法模拟父类的某些方法，为了单元测试用例覆盖率达到70%以上，进行了拆分。具体的修改为：

Before

public SendResult doSendMessageInTransaction(final Message msg, long timeout, String xid, long branchId)
	throws MQClientException {
	//...
	try {
		sendResult = super.send(msg, timeout);
	} catch (Exception e) {
		throw new MQClientException("send message Exception", e);
	}
	//...
}

After

public SendResult doSendMessageInTransaction(final Message msg, long timeout, String xid, long branchId)
	throws MQClientException {
	//...
	try {
		sendResult = superSend(msg, timeout);
	} catch (Exception e) {
		throw new MQClientException("send message Exception", e);
	}
	//...
}
public SendResult superSend(Message msg, long timeout)
	throws MQClientException, MQBrokerException, RemotingException, InterruptedException {
	return super.send(msg, timeout);
}

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 51.93%. Comparing base (1e26c91) to head (4598d9f).

Files with missing lines	Patch %	Lines
...he/seata/integration/rocketmq/SeataMQProducer.java	85.71%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #6927      +/-   ##
============================================
+ Coverage     51.80%   51.93%   +0.12%     
- Complexity     6462     6473      +11     
============================================
  Files          1122     1122              
  Lines         39857    39860       +3     
  Branches       4668     4670       +2     
============================================
+ Hits          20649    20701      +52     
+ Misses        17248    17194      -54     
- Partials       1960     1965       +5     

Files with missing lines	Coverage Δ
...he/seata/integration/rocketmq/SeataMQProducer.java	85.96% <85.71%> (+63.74%)	⬆️

... and 6 files with indirect coverage changes