[Studio] fix: implement namespace listing

[Loyal-Young]

Summary

• implement the previously unsupported namespace listing operation behind GET /api/namespaces
• derive unique namespaces from topic metadata while retaining cluster identity and skipping blank namespaces
• return namespace records in deterministic name and cluster order, with service-level coverage

Validation

• git diff --check
• The current Windows host does not have Java, Maven, or a Maven Wrapper available, so the new MetadataServiceTest could not be executed locally.

[lizhimins]

Closing as deferred: the current version does not yet support Namespace (the Namespace frontend page #478 was also closed). We'll revisit namespace listing when Namespace support is on the roadmap. Thanks @Loyal-Young!