Fix charge-backed discount invoice edits

.agents/skills/e2e/SKILL.md

@@ -12,6 +12,8 @@ You are helping the user write OpenMeter end-to-end tests that run against a liv
 
 This is the black-box layer. Unlike the `/test` skill (which covers in-process unit/integration/service tests using `testutils.TestEnv` + `testutils.InitPostgresDB`), e2e tests hit the wire format: JSON in, JSON out, status codes, problem+json error bodies. Use this skill when the value of the test comes from exercising the HTTP contract, the OpenAPI binder, or cross-service behavior.
 
+General test style from `AGENTS.md` and the `/test` skill still applies. Keep this skill as e2e-specific guidance, not a parallel set of test conventions.
+
 ## Two styles, same package
 
 Both live in `e2e/` and share the build tag, environment, and skip-when-unset convention. Pick by what the endpoint under test offers:
@@ -48,6 +50,10 @@ Notes:
 
 ## Shared conventions (both styles)
 
+### Helper functions
+
+Prefer a single explicit test body over single-use setup wrappers. Do not add helper functions for e2e setup, conversion, or assertions unless the helper is used by at least two tests in the same package or its name captures non-obvious domain semantics that would otherwise be easy to miss.
+
 ### Unique fixture keys
 
 The docker-compose DB is shared across re-runs and parallel tests. Fixed keys collide. Always generate keys with a suffix:

AGENTS.md

@@ -147,7 +147,7 @@ In tests, prefer `t.Context()` when a `testing.T` or `testing.TB` is available i
 
 Prefer one consistent test harness style over mixed ad hoc structures. Use production-backed paths, such as rating-backed or service-backed fixtures, when the real path can express the scenario; keep hand-assembled fixtures for cases that cannot be produced realistically. If a behavior is a suite-wide rule, hardcode it into the shared harness instead of exposing it as per-test knobs.
 
-Avoid redundant test helpers and duplicate setup paths. Prefer parameterizing one helper over maintaining near-identical helpers, use literal helper names that state exactly what they do, and inline two-line single-use helpers that do not need `defer`. Clean up dead test helpers immediately after refactors.
+Avoid redundant test helpers and duplicate setup paths. Prefer parameterizing one helper over maintaining near-identical helpers, use literal helper names that state exactly what they do, and inline single-use helpers that only wrap setup, conversion, or assertions even when the test becomes longer. Add a test helper only when it is used by at least two tests in the same package or when the helper name captures non-obvious domain semantics that would otherwise be easy to miss. Clean up dead test helpers immediately after refactors.
 
 For service and lifecycle subtests, start each subtest body with concise intent comments when the scenario is non-trivial:
 

api/client/javascript/src/client/schemas.ts

@@ -7725,7 +7725,10 @@ export interface components {
        *     When null, the feature or service is free.
        */
       price: components['schemas']['RateCardUsageBasedPrice'] | null
-      /** @description The discounts that are applied to the line. */
+      /**
+       * @deprecated
+       * @description The discounts that are applied to the line.
+       */
       discounts?: components['schemas']['BillingDiscounts']
     }
     /** @description InvoiceWorkflowInvoicingSettingsReplaceUpdate represents the update model for the invoicing settings of an invoice workflow. */

api/openapi.cloud.yaml

@@ -19773,6 +19773,7 @@ components:
           allOf:
             - $ref: '#/components/schemas/BillingDiscounts'
           description: The discounts that are applied to the line.
+          deprecated: true
       description: InvoiceUsageBasedRateCard represents the rate card (intent) for an usage-based line.
     InvoiceWorkflowInvoicingSettingsReplaceUpdate:
       type: object

api/openapi.yaml

@@ -20471,6 +20471,7 @@ components:
           allOf:
             - $ref: '#/components/schemas/BillingDiscounts'
           description: The discounts that are applied to the line.
+          deprecated: true
       description: InvoiceUsageBasedRateCard represents the rate card (intent) for an usage-based line.
     InvoiceWorkflowInvoicingSettingsReplaceUpdate:
       type: object

api/spec/packages/legacy/src/billing/invoices/invoice.tsp

@@ -656,6 +656,7 @@ model InvoiceUsageBasedRateCard {
   /**
    * The discounts that are applied to the line.
    */
+  #deprecated "V1 invoice responses do not expose persisted rate card discounts"
   @visibility(Lifecycle.Read, Lifecycle.Create, Lifecycle.Update)
   discounts?: BillingDiscounts;
 }

api/v3/handlers/customers/charges/convert.go

@@ -298,7 +298,7 @@ func toAPIBillingPriceTier(t productcatalog.PriceTier, _ int) api.BillingPriceTi
 }
 
 // convertFlatFeeDiscounts maps the optional percentage discount to the anonymous API struct.
-func convertFlatFeeDiscounts(pd *productcatalog.PercentageDiscount) *api.BillingChargeFlatFeeDiscounts {
+func convertFlatFeeDiscounts(pd *billing.PercentageDiscount) *api.BillingChargeFlatFeeDiscounts {
 	if pd == nil {
 		return nil
 	}
@@ -307,7 +307,7 @@ func convertFlatFeeDiscounts(pd *productcatalog.PercentageDiscount) *api.Billing
 }
 
 // convertUsageBasedDiscounts maps usage-based discounts to the API type.
-func convertUsageBasedDiscounts(d productcatalog.Discounts) *api.BillingRateCardDiscounts {
+func convertUsageBasedDiscounts(d billing.Discounts) *api.BillingRateCardDiscounts {
 	if d.Percentage == nil && d.Usage == nil {
 		return nil
 	}
@@ -476,11 +476,15 @@ func convertFlatFeeChargeAPIToIntent(customerID string, flatFee api.CreateCharge
 		metadata = models.Metadata(*flatFee.Labels)
 	}
 
-	var discount *productcatalog.PercentageDiscount
+	var discount *billing.PercentageDiscount
 	if flatFee.Discounts != nil && flatFee.Discounts.Percentage != nil {
-		discount = &productcatalog.PercentageDiscount{
-			Percentage: models.NewPercentage(float64(lo.FromPtr(flatFee.Discounts.Percentage))),
-		}
+		discount = billing.Discounts{
+			Percentage: &billing.PercentageDiscount{
+				PercentageDiscount: productcatalog.PercentageDiscount{
+					Percentage: models.NewPercentage(float64(lo.FromPtr(flatFee.Discounts.Percentage))),
+				},
+			},
+		}.UpsertCorrelationIDs().Percentage
 	}
 
 	var proRating productcatalog.ProRatingConfig
@@ -537,22 +541,27 @@ func convertUsageBaseChargeAPIToIntent(customerID string, usageBasedFee api.Crea
 		metadata = models.Metadata(*usageBasedFee.Labels)
 	}
 
-	var discounts productcatalog.Discounts
+	var discounts billing.Discounts
 	if usageBasedFee.Discounts != nil {
 		if usageBasedFee.Discounts.Percentage != nil {
-			discounts.Percentage = &productcatalog.PercentageDiscount{
-				Percentage: models.NewPercentage(float64(lo.FromPtr(usageBasedFee.Discounts.Percentage))),
+			discounts.Percentage = &billing.PercentageDiscount{
+				PercentageDiscount: productcatalog.PercentageDiscount{
+					Percentage: models.NewPercentage(float64(lo.FromPtr(usageBasedFee.Discounts.Percentage))),
+				},
 			}
 		}
 		if usageBasedFee.Discounts.Usage != nil {
 			quantity, err := alpacadecimal.NewFromString(lo.FromPtr(usageBasedFee.Discounts.Usage))
 			if err != nil {
 				return zero, fmt.Errorf("invalid usage discount quantity: %w", err)
 			}
-			discounts.Usage = &productcatalog.UsageDiscount{
-				Quantity: quantity,
+			discounts.Usage = &billing.UsageDiscount{
+				UsageDiscount: productcatalog.UsageDiscount{
+					Quantity: quantity,
+				},
 			}
 		}
+		discounts = discounts.UpsertCorrelationIDs()
 	}
 
 	price, err := plans.FromAPIBillingPrice(usageBasedFee.Price, lo.ToPtr(api.BillingPricePaymentTermInArrears))