[Studio][Bug] Client connection listing returns 502 when every consumer group is offline

## Problem

The Clients page (`GET /api/clients`) fails with HTTP 502 "Failed to query consumer connections from all groups" whenever every non-system consumer group of the instance is offline — a normal state (consumer applications stopped, dev/test cluster, nightly windows) — instead of returning an empty connection list. The producer connections already collected in the same request are discarded too. The explicit producer-group lookup (`GET /api/producer/connection?...&producerGroup=G`) fails the same way for an offline group.

## Evidence

`rocketmq-tools` 5.5.0 (pinned by `server/pom.xml`), verified via `javap -c` on `DefaultMQAdminExtImpl`:

- `examineConsumerConnectionInfo(String)` throws `new MQClientException(206, "Not found the consumer group connection")` when `ConsumerConnection.getConnectionSet().isEmpty()`.
- `examineProducerConnectionInfo(String, String)` throws `new MQClientException("Not found the producer group connection", null)` for an empty connection set.

`RocketMQClientProvider.findConsumerConnections` (`server/src/main/java/org/apache/rocketmq/studio/provider/apache/RocketMQClientProvider.java:313-334`) counts those per-group answers as scan failures, so an all-offline cluster is indistinguishable from all brokers being unreachable and trips the all-failed guard at line 332. The provider's own convention is that normal "nothing here" states return empty, not 502 (comment at `:175-177`, `isTopicNotExist` handling).

## Impact

Any cluster whose consumers are simply not running shows a hard failure on the Clients page and loses the producer connections fetched in the same request, instead of an empty/partial list.

## Expected behavior

An offline group is a normal "no connections right now" answer: the consumer scan should treat it as a successful empty result (all-offline ⇒ empty list), and the explicit producer-group query should return an empty list. Genuine broker failures keep returning 502, and partial scans keep returning partial results.

## Related work

This refines the all-failed heuristic introduced for #1142/#1143 (distinguish failed scans from empty results); it does not change it for real failures. The same "normal absence → empty" convention was already applied to NO_MESSAGE key queries (#3305) and topic-route absence (#3359).

## PR

#4002

Start in server/src/main/java/org/apache/rocketmq/studio/provider/apache/RocketMQClientProvider.java:313-334 and review the convention described at lines 175-177. Compare the client and producer connection endpoints, then inspect PR #4002 and verify that offline groups produce empty or partial results while genuine broker failures still return 502.

Written by the indexing model from the issue text.