[Bug] Proxy getConsumerListByGroup NPEs when consumer group is offline

### What happened?

`ConsumerManagerActivity.getConsumerListByGroup` calls `messagingProcessor.getConsumerGroupInfo(...)` and immediately dereferences the returned `ConsumerGroupInfo`. When the requested group is not online, the processor can return `null`, causing a `NullPointerException`.

`getConsumerConnectionList` in the same activity already handles this case by returning `CONSUMER_NOT_ONLINE`, so the two client-management queries currently have inconsistent behavior.

### Why this matters

The Proxy client-management path is part of the Studio/Proxy Admin foundation. Querying online consumers for an offline group should return a deterministic client-facing response instead of an internal error.

### Expected behavior

`GET_CONSUMER_LIST_BY_GROUP` should return `CONSUMER_NOT_ONLINE` with a clear remark when the group is not online.

### Scope

RocketMQ Studio track 2 / Proxy client-management hardening.

Start at ConsumerManagerActivity.getConsumerListByGroup and inspect how it uses messagingProcessor.getConsumerGroupInfo(...). Compare its offline-group handling with getConsumerConnectionList, then verify that an offline group returns CONSUMER_NOT_ONLINE with a clear remark instead of causing an internal error.

Written by the indexing model from the issue text.