feat(payment): add complete payment system with multi-provider support

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.

backend/internal/payment/provider/alipay_test.go

@@ -0,0 +1,132 @@
+//go:build unit
+
+package provider
+
+import (
+	"errors"
+	"strings"
+	"testing"
+)
+
+func TestIsTradeNotExist(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		err  error
+		want bool
+	}{
+		{
+			name: "nil error returns false",
+			err:  nil,
+			want: false,
+		},
+		{
+			name: "error containing ACQ.TRADE_NOT_EXIST returns true",
+			err:  errors.New("alipay: sub_code=ACQ.TRADE_NOT_EXIST, sub_msg=交易不存在"),
+			want: true,
+		},
+		{
+			name: "error not containing the code returns false",
+			err:  errors.New("alipay: sub_code=ACQ.SYSTEM_ERROR, sub_msg=系统错误"),
+			want: false,
+		},
+		{
+			name: "error with only partial match returns false",
+			err:  errors.New("ACQ.TRADE_NOT"),
+			want: false,
+		},
+		{
+			name: "error with exact constant value returns true",
+			err:  errors.New(alipayErrTradeNotExist),
+			want: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			got := isTradeNotExist(tt.err)
+			if got != tt.want {
+				t.Errorf("isTradeNotExist(%v) = %v, want %v", tt.err, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestNewAlipay(t *testing.T) {
+	t.Parallel()
+
+	validConfig := map[string]string{
+		"appId":      "2021001234567890",
+		"privateKey": "MIIEvQIBADANBgkqhkiG9w0BAQEFAASC...",
+	}
+
+	// helper to clone and override config fields
+	withOverride := func(overrides map[string]string) map[string]string {
+		cfg := make(map[string]string, len(validConfig))
+		for k, v := range validConfig {
+			cfg[k] = v
+		}
+		for k, v := range overrides {
+			cfg[k] = v
+		}
+		return cfg
+	}
+
+	tests := []struct {
+		name      string
+		config    map[string]string
+		wantErr   bool
+		errSubstr string
+	}{
+		{
+			name:    "valid config succeeds",
+			config:  validConfig,
+			wantErr: false,
+		},
+		{
+			name:      "missing appId",
+			config:    withOverride(map[string]string{"appId": ""}),
+			wantErr:   true,
+			errSubstr: "appId",
+		},
+		{
+			name:      "missing privateKey",
+			config:    withOverride(map[string]string{"privateKey": ""}),
+			wantErr:   true,
+			errSubstr: "privateKey",
+		},
+		{
+			name:    "nil config map returns error for appId",
+			config:  map[string]string{},
+			wantErr: true,
+			errSubstr: "appId",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			got, err := NewAlipay("test-instance", tt.config)
+			if tt.wantErr {
+				if err == nil {
+					t.Fatal("expected error, got nil")
+				}
+				if tt.errSubstr != "" && !strings.Contains(err.Error(), tt.errSubstr) {
+					t.Errorf("error %q should contain %q", err.Error(), tt.errSubstr)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if got == nil {
+				t.Fatal("expected non-nil Alipay instance")
+			}
+			if got.instanceID != "test-instance" {
+				t.Errorf("instanceID = %q, want %q", got.instanceID, "test-instance")
+			}
+		})
+	}
+}