test(1306): pin usage serialization, snapshot cache invalidation, and listing send classification (#5000)

test(1306): pin Some(0) usage serialization, snapshot cache invalidation, and listing send classification

Follow-up test hardening for the merged admin-usage-snapshot work
(#4979/#4980/#4981/#4982, rustfs/backlog#1306). Tests only; no production
behavior change.

- B-1 madmin: pin that a scanned-but-empty bucket (Some(0)) serializes usage
  stats as zeros, staying distinct from the no-snapshot (None) omitted case.
- B-2 gating: revert detector proving save_data_usage_in_backend invalidates
  the 30s snapshot cache so a fresh save is visible to the next cached read.
- A-1 list_objects: pin that a successful gather_results send is never
  misclassified as ConsumerGone (correct state + err sentinel delivered), and
  document the wrapper Err arm invariant. A full wrapper-level producer-error
  integration test is deferred as it needs the fake-disk list harness.

crates/madmin/src/user.rs

@@ -771,6 +771,33 @@ mod tests {
         assert_eq!(value["object_versions_histogram"]["SINGLE_VERSION"], 7);
     }
 
+    /// Wire pin (rustfs/backlog#1306): a scanned-but-empty bucket carries
+    /// Some(0) usage stats that must serialize as zeros, staying distinct from
+    /// the no-snapshot (None) case that is omitted. Guards against replacing
+    /// `Option::is_none` with a predicate that also skips zero values.
+    #[test]
+    fn bucket_access_info_serializes_zero_usage_stats() {
+        let info = BucketAccessInfo {
+            name: "empty-snapshot".to_string(),
+            size: Some(0),
+            objects: Some(0),
+            object_sizes_histogram: Some(HashMap::new()),
+            object_versions_histogram: Some(HashMap::new()),
+            ..Default::default()
+        };
+
+        let value = serde_json::to_value(&info).unwrap();
+        let obj = value.as_object().unwrap();
+        assert!(obj.contains_key("size"));
+        assert!(obj.contains_key("objects"));
+        assert!(obj.contains_key("object_sizes_histogram"));
+        assert!(obj.contains_key("object_versions_histogram"));
+        assert_eq!(value.get("size"), Some(&serde_json::json!(0)));
+        assert_eq!(value["objects"], 0);
+        assert_eq!(value["object_sizes_histogram"], serde_json::json!({}));
+        assert_eq!(value["object_versions_histogram"], serde_json::json!({}));
+    }
+
     #[test]
     fn test_account_status_try_from_invalid() {
         let result = AccountStatus::try_from("invalid");