[ISSUE #9341]: Fix Namespace v2 not taking effect

[JimmyWang6]

Which Issue(s) This PR Fixes

Fixes #9341

Currently, namespace v2 is not actually taking effect for the RocketMQ broker, which has caused some confusion among users as shown below:
apache/rocketmq-spring#722
#9341

Therefore, for the sake of backward compatibility, it is advisable to set the namespace field—despite it being marked as deprecated—to avoid confusing users.

How Did You Test This Change?

1、setNamespaceV2.
2、produce some messages for Topic A under namespace A .
3、consumer from the some Topic A under namespace B.
4、No message received from consumer.

[yx9o]

I think it would be better to implement the namespaceV2 feature. This change is prone to misunderstanding. Why not just use namespace directly? You can try to implement this feature.

[JimmyWang6]

@yx9o Thanks for your reply! I think the main reason is that there aren't any documents about why namespace v2 was introduced and what problems it solves compared to v1. From my perspective, this appears to be an unfinished feature, so exposing its configuration to clients may not be ideal. However, I'd be glad to help implement this feature if I could.