fix: rate card reject unit config

Author: tothandras

api/v3/handlers/plans/convert.go

@@ -527,6 +527,14 @@ func FromAPIBillingPlanPhase(p api.BillingPlanPhase) (productcatalog.Phase, erro
 }
 
 func FromAPIBillingRateCard(rc api.BillingRateCard) (productcatalog.RateCard, error) {
+	// unit_config is read-only: it is synthesized from v1 dynamic and package
+	// prices on the response path. Accepting it on create/update would silently
+	// drop the original v1 price semantics, so reject it explicitly until v3
+	// authoring is designed.
+	if rc.UnitConfig != nil {
+		return nil, models.NewGenericValidationError(fmt.Errorf("unit_config is not accepted on create or update for rate card %q", rc.Key))
+	}
+
 	priceType, err := rc.Price.Discriminator()
 	if err != nil {
 		return nil, fmt.Errorf("failed to read price type: %w", err)

api/v3/handlers/plans/convert_test.go

@@ -1300,6 +1300,31 @@ func TestToRateCard(t *testing.T) {
 		require.NoError(t, err)
 		assert.Equal(t, productcatalog.VolumeTieredPrice, tiered.Mode)
 	})
+
+	t.Run("rejects unit_config on create/update", func(t *testing.T) {
+		var price api.BillingPrice
+		require.NoError(t, price.FromBillingPriceUnit(api.BillingPriceUnit{Amount: "1", Type: "unit"}))
+
+		bc := api.ISO8601Duration("P1M")
+
+		rc := api.BillingRateCard{
+			Key:            "tokens",
+			Name:           "Tokens",
+			Price:          price,
+			BillingCadence: &bc,
+			UnitConfig: &api.BillingUnitConfig{
+				Operation:        api.BillingUnitConfigOperationMultiply,
+				ConversionFactor: "1.2",
+			},
+		}
+
+		_, err := FromAPIBillingRateCard(rc)
+		require.Error(t, err)
+		var validationErr *models.GenericValidationError
+		assert.ErrorAs(t, err, &validationErr)
+		assert.Contains(t, err.Error(), "unit_config")
+		assert.Contains(t, err.Error(), "tokens")
+	})
 }
 
 func TestToBillingPrice(t *testing.T) {