fix: harden single-machine load handling

loadtest/im-pressure/httpapi.go

@@ -269,6 +269,11 @@ type groupMembersResponse struct {
 }
 
 func (a *apiClient) countGroupMembers(ctx context.Context, account AccountRecord, topic string) (int, error) {
+	members, err := a.groupMemberIDs(ctx, account, topic)
+	return len(members), err
+}
+
+func (a *apiClient) groupMemberIDs(ctx context.Context, account AccountRecord, topic string) (map[string]bool, error) {
 	cursor := ""
 	seen := make(map[string]bool)
 	for {
@@ -278,7 +283,7 @@ func (a *apiClient) countGroupMembers(ctx context.Context, account AccountRecord
 		}
 		var page groupMembersResponse
 		if err := a.do(ctx, http.MethodGet, path, account.Token, "", nil, &page); err != nil {
-			return 0, err
+			return nil, err
 		}
 		for _, item := range page.Items {
 			seen[item.UserID] = true
@@ -287,11 +292,11 @@ func (a *apiClient) countGroupMembers(ctx context.Context, account AccountRecord
 			break
 		}
 		if page.NextCursor == cursor {
-			return 0, fmt.Errorf("group member cursor did not advance: %s", cursor)
+			return nil, fmt.Errorf("group member cursor did not advance: %s", cursor)
 		}
 		cursor = page.NextCursor
 	}
-	return len(seen), nil
+	return seen, nil
 }
 
 func queryInt(value string, fallback int) int {