Follow-up to #2698. The databases/list call posted with no request body,
which the Milvus demo cluster used during testing accepts. Production
Milvus is stricter: a bodyless POST returns HTTP 200 with
`{"code":1801,...}` and omits the `data` field. Since the success check
only inspects the HTTP status, list_milvus_databases() parsed zero names
and silently fell back to `["default"]`, hiding every non-default database
in the sidebar.
Send an empty JSON object `{}` as the body, matching every other Milvus
v2 endpoint. With the body, the endpoint returns code 0 and the full
database list on both demo and production clusters.
Ref #2693
Co-authored-by: 朱豪豪 <zhu.hh.sz@belle.com.cn>