fix(milvus): send empty JSON body to databases/list endpoint

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>

crates/dbx-core/src/db/vector_driver.rs

@@ -183,7 +183,11 @@ pub async fn list_databases(client: &VectorClient) -> Result<Vec<String>, String
 async fn list_milvus_databases(client: &VectorClient) -> Result<Vec<String>, String> {
     // Older Milvus versions (pre-2.2) do not expose the databases endpoint; fall back to "default"
     // so the connection stays browsable instead of failing the whole tree load.
-    let body = match send_json(client.post("/v2/vectordb/databases/list"), "Milvus").await {
+    //
+    // The endpoint rejects a bodyless POST with `{"code":1801,...}` (HTTP 200, no `data` field),
+    // so send an empty JSON object like every other Milvus v2 endpoint.
+    let body = match send_json(client.post("/v2/vectordb/databases/list").json(&serde_json::json!({})), "Milvus").await
+    {
         Ok(body) => body,
         Err(_) => return Ok(vec!["default".to_string()]),
     };