fix: preserve billing response shape

backend/internal/httpapi/billing_test.go

@@ -28,6 +28,15 @@ func TestBillingMemberRoutesUseRefreshedSessionScope(t *testing.T) {
 	}
 }
 
+func TestBillingOverviewResponsesEncodeEmptyCollectionsAsArrays(t *testing.T) {
+	service := billing.NewService(&emptyBillingStore{}, nil)
+	member := billingTestHandler(t, identity.Session{AuthMode: identity.AuthModeUser, User: identity.User{ID: "user", ClientID: "platform", OrganizationID: "org", Role: "user"}}, service, nil)
+	admin := billingTestHandler(t, identity.Session{AuthMode: identity.AuthModeAdmin, User: identity.User{ID: "root", ClientID: "platform", Role: "super_admin"}}, service, nil)
+
+	assertJSONArrays(t, serveJSON(t, member, http.MethodGet, "/api/billing", nil), "ledger")
+	assertJSONArrays(t, serveJSON(t, admin, http.MethodGet, "/api/admin/billing", nil), "organizations", "members", "ledger", "priceRules")
+}
+
 func TestBillingQuotePreparesProviderAndModelFromServerOwnedEngineTargets(t *testing.T) {
 	service := &billingHTTPServiceStub{}
 	h := billingTestHandler(t, identity.Session{User: identity.User{ID: "db-user", ClientID: "platform", OrganizationID: "db-org", Role: "user"}}, service, nil)
@@ -165,6 +174,39 @@ func (s *billingAccountStoreStub) Save(_ context.Context, value billing.AccountC
 	return nil
 }
 
+type emptyBillingStore struct{}
+
+func (*emptyBillingStore) BillingWallet(context.Context, string) (billing.Wallet, error) {
+	return billing.Wallet{}, nil
+}
+func (*emptyBillingStore) BillingWallets(context.Context) ([]billing.Wallet, error) {
+	return nil, nil
+}
+func (*emptyBillingStore) BillingLedger(context.Context, string, string, int) ([]billing.LedgerEntry, error) {
+	return nil, nil
+}
+func (*emptyBillingStore) BillingOrganizations(context.Context) ([]billing.Organization, error) {
+	return nil, nil
+}
+func (*emptyBillingStore) BillingMembers(context.Context) ([]billing.Member, error) {
+	return nil, nil
+}
+func (*emptyBillingStore) BillingOrganizationExists(context.Context, string) (bool, error) {
+	return false, nil
+}
+func (*emptyBillingStore) ListBillingPriceRules(context.Context, bool) ([]billing.PriceRule, error) {
+	return nil, nil
+}
+func (*emptyBillingStore) GetBillingPriceRule(context.Context, string) (*billing.PriceRule, error) {
+	return nil, nil
+}
+func (*emptyBillingStore) UpdateBillingPriceRule(context.Context, string, billing.PricePatch) (*billing.PriceRule, error) {
+	return nil, nil
+}
+func (*emptyBillingStore) PostBillingWalletEntry(context.Context, billing.WalletPostParams) (billing.WalletPosting, error) {
+	return billing.WalletPosting{}, nil
+}
+
 func billingTestHandler(t *testing.T, session identity.Session, service BillingHTTPService, accounts BillingAccountStore) http.Handler {
 	t.Helper()
 	authorizer, err := NewPlatformAuthorizer(AuthState{Required: true, Configured: true}, &fixedSessionResolver{session: session})
@@ -192,3 +234,19 @@ func serveJSON(t *testing.T, h http.Handler, method, path string, body any) *htt
 	h.ServeHTTP(response, req)
 	return response
 }
+
+func assertJSONArrays(t *testing.T, response *httptest.ResponseRecorder, fields ...string) {
+	t.Helper()
+	if response.Code != http.StatusOK {
+		t.Fatalf("status=%d body=%s", response.Code, response.Body.String())
+	}
+	var payload map[string]json.RawMessage
+	if err := json.Unmarshal(response.Body.Bytes(), &payload); err != nil {
+		t.Fatalf("decode response: %v", err)
+	}
+	for _, field := range fields {
+		if string(payload[field]) != "[]" {
+			t.Errorf("%s = %s, want [] (body=%s)", field, payload[field], response.Body.String())
+		}
+	}
+}