test(payment): add unit tests for payment audit fixes + allow empty supported_types

Tests (1033 new lines, 100% coverage on modified functions):
- amount.go: YuanToFen/FenToYuan with precision edge cases
- wxpay: mapWxState, wxSV, formatPEM, NewWxpay validation
- alipay: isTradeNotExist, NewAlipay validation
- webhook: writeSuccessResponse (wxpay JSON, stripe empty, others text)
- config: validateProviderRequest, isSensitiveConfigField, joinTypes
- fulfillment: resolveRedeemAction idempotency logic

Business logic changes:
- Allow empty supported_types on provider instances
- Block removing payment types when instance has pending orders
- Extract resolveRedeemAction as testable pure function

backend/internal/payment/provider/wxpay_test.go

@@ -156,7 +156,6 @@ func TestNewWxpay(t *testing.T) {
 		"apiV3Key":    "12345678901234567890123456789012", // exactly 32 bytes
 		"publicKey":   "fake-public-key",
 		"publicKeyId": "key-id-001",
-		"certSerial":  "SERIAL001",
 	}
 
 	// helper to clone and override config fields

backend/internal/service/payment_fulfillment.go

@@ -146,20 +146,46 @@ func (s *PaymentService) ExecuteBalanceFulfillment(ctx context.Context, oid int6
 	return nil
 }
 
+// redeemAction represents the idempotency decision for balance fulfillment.
+type redeemAction int
+
+const (
+	// redeemActionCreate: code does not exist — create it, then redeem.
+	redeemActionCreate redeemAction = iota
+	// redeemActionRedeem: code exists but is unused — skip creation, redeem only.
+	redeemActionRedeem
+	// redeemActionSkipCompleted: code exists and is already used — skip to mark completed.
+	redeemActionSkipCompleted
+)
+
+// resolveRedeemAction decides the idempotency action based on an existing redeem code lookup.
+// existing is the result of GetByCode; lookupErr is the error from that call.
+func resolveRedeemAction(existing *RedeemCode, lookupErr error) redeemAction {
+	if existing == nil || lookupErr != nil {
+		return redeemActionCreate
+	}
+	if existing.IsUsed() {
+		return redeemActionSkipCompleted
+	}
+	return redeemActionRedeem
+}
+
 func (s *PaymentService) doBalance(ctx context.Context, o *dbent.PaymentOrder) error {
 	// Idempotency: check if redeem code already exists (from a previous partial run)
-	existing, _ := s.redeemService.GetByCode(ctx, o.RechargeCode)
-	if existing != nil {
-		if existing.IsUsed() {
-			// Code already created and redeemed — just mark completed
-			return s.markCompleted(ctx, o, "RECHARGE_SUCCESS")
-		}
-		// Code exists but unused — skip creation, proceed to redeem
-	} else {
+	existing, lookupErr := s.redeemService.GetByCode(ctx, o.RechargeCode)
+	action := resolveRedeemAction(existing, lookupErr)
+
+	switch action {
+	case redeemActionSkipCompleted:
+		// Code already created and redeemed — just mark completed
+		return s.markCompleted(ctx, o, "RECHARGE_SUCCESS")
+	case redeemActionCreate:
 		rc := &RedeemCode{Code: o.RechargeCode, Type: RedeemTypeBalance, Value: o.Amount, Status: StatusUnused}
 		if err := s.redeemService.CreateCode(ctx, rc); err != nil {
 			return fmt.Errorf("create redeem code: %w", err)
 		}
+	case redeemActionRedeem:
+		// Code exists but unused — skip creation, proceed to redeem
 	}
 	if _, err := s.redeemService.Redeem(ctx, o.UserID, o.RechargeCode); err != nil {
 		return fmt.Errorf("redeem balance: %w", err)