feat(v2): 支付宝 adapter 落地 RefundingProvider(TradeRefund 同步退款,out_request_no=refund_id 幂等)

internal/provider/alipay/alipay.go

@@ -31,12 +31,36 @@ func (p *Provider) Method() string { return "alipay" }
 func (p *Provider) Capabilities() provider.Capabilities {
 	return provider.Capabilities{
 		RenderTypes:      []provider.RenderType{provider.RenderRedirect},
-		SupportsRefund:   false, // 退款 P4
+		SupportsRefund:   true, // P4:alipay.trade.refund 同步退款
 		SettleCurrencies: []string{"CNY"},
 		Regions:          []string{"cn"},
 	}
 }
 
+// Refund 调 alipay.trade.refund(同步接口)。providerRef=out_trade_no(alipay 归位键),
+// refundID 作 out_request_no(部分退款必传且须唯一稳定 → 幂等)。IsFailure()==false 即成功
+// (FundChange=N 表重复退款已幂等,仍算成功)。
+func (p *Provider) Refund(ctx context.Context, providerRef, refundID string, amountMinor int64, reason string) (string, provider.PaidStatus, error) {
+	amt, err := money.Format(amountMinor, "CNY")
+	if err != nil {
+		return "", provider.PaidFailed, err
+	}
+	rsp, err := p.client.TradeRefund(ctx, sw.TradeRefund{
+		OutTradeNo:   providerRef,
+		RefundAmount: amt,
+		RefundReason: reason,
+		OutRequestNo: refundID,
+	})
+	if err != nil {
+		return "", provider.PaidFailed, fmt.Errorf("alipay: 退款请求失败: %w", err)
+	}
+	if rsp.IsFailure() {
+		return "", provider.PaidFailed, fmt.Errorf("alipay: 退款被拒: %w", rsp.Error)
+	}
+	// 退款在支付宝侧以 out_request_no 定位;refundRef 回传我方退款单号(= out_request_no)。
+	return refundID, provider.PaidSucceeded, nil
+}
+
 func (p *Provider) Create(_ context.Context, req provider.CreateRequest) (*provider.Session, error) {
 	if req.Currency != "CNY" {
 		return nil, fmt.Errorf("alipay: 仅支持 CNY, got %s", req.Currency)