[ISSUE #5092] unit test added for ``EtcdCustomService``

[denizOgut]

Fixes https://github.com/apache/eventmesh/issues/5092

Motivation Increase test coverage

Modifications Added new tests for EtcdCustomService.java located eventmesh-meta/eventmesh-meta-etcd/src/main/java/org/apache/eventmesh/meta/etcd/service/EtcdCustomService.java

Documentation Does this pull request introduce a new feature? (no) If yes, how is the feature documented? (not applicable) If a feature is not applicable for documentation, explain why? If a feature is not documented yet in this PR, please create a followup issue for adding the documentation