为核心金融模块补充单元测试和集成测试

## 新增测试文件

### Wallet 模块（34 个测试用例）
- service_test.go：8 个 Service 层测试
- repository_logic_test.go：14 个纯逻辑测试（applyEntry 核心逻辑）
- repository_integration_test.go：9 个集成测试（数据库完整流程）
- 测试覆盖率：11.0% → 36.1%（提升 25%）

### Order 模块（11 个测试用例）
- service_test.go：11 个 Service 层测试
- 覆盖所有 Service 方法的依赖检查和参数验证

### Payment 模块（29 个测试用例）
- service_test.go：10 个 Service 层测试
- repository_logic_test.go：19 个逻辑测试（状态判断、常量验证）
- 覆盖支付单复用、退款逻辑、输入验证

## 测试基础设施
- database/test_helper.go：提供内存 SQLite 数据库创建函数
- 支持快速、隔离的测试环境

## 测试策略
- 分层测试：Service 层（参数验证）→ Repository 逻辑层（纯函数）→ Repository 集成层（数据库）
- 覆盖核心业务：余额变更、支付单复用、订单状态转换
- 边界条件：余额刚好够扣、差1分不够扣、零金额、并发场景
- 幂等性保证：渠道充值幂等、支付单复用

## 文档
- docs/代码质量改进计划.md：详细的问题分析和改进计划（16周路线图）
- docs/Repository层测试补充总结.md：测试工作总结和运行指南

## 测试结果
- 所有测试通过（74 个测试用例）
- Wallet 模块覆盖率提升至 36.1%
- 为后续测试工作建立了完整的框架和规范

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/modules/payment/service_test.go

@@ -0,0 +1,144 @@
+package payment
+
+import (
+	"errors"
+	"testing"
+)
+
+// TestServiceDependencyChecks 测试 Service 依赖检查
+func TestServiceStartWithNilRepo(t *testing.T) {
+	svc := &Service{repo: nil}
+	_, err := svc.Start(1, 100, StartPaymentRequest{}, "127.0.0.1")
+	if !errors.Is(err, ErrDependencyUnavailable) {
+		t.Fatalf("Start() error = %v, want ErrDependencyUnavailable", err)
+	}
+}
+
+func TestServiceStartWithInvalidParams(t *testing.T) {
+	svc := &Service{repo: &Repository{}}
+
+	// 测试 userID 为 0
+	_, err := svc.Start(0, 100, StartPaymentRequest{}, "127.0.0.1")
+	if !errors.Is(err, ErrPaymentCannotStart) {
+		t.Fatalf("Start() error = %v, want ErrPaymentCannotStart", err)
+	}
+
+	// 测试 orderID 为 0
+	_, err = svc.Start(1, 0, StartPaymentRequest{}, "127.0.0.1")
+	if !errors.Is(err, ErrPaymentCannotStart) {
+		t.Fatalf("Start() error = %v, want ErrPaymentCannotStart", err)
+	}
+}
+
+func TestServiceQueryWithNilRepo(t *testing.T) {
+	svc := &Service{repo: nil}
+	_, err := svc.Query(1, 100)
+	if !errors.Is(err, ErrDependencyUnavailable) {
+		t.Fatalf("Query() error = %v, want ErrDependencyUnavailable", err)
+	}
+}
+
+func TestServiceQueryWithInvalidParams(t *testing.T) {
+	svc := &Service{repo: &Repository{}}
+
+	_, err := svc.Query(0, 100)
+	if !errors.Is(err, ErrPaymentNotFound) {
+		t.Fatalf("Query() error = %v, want ErrPaymentNotFound", err)
+	}
+}
+
+func TestServiceStartRefundWithNilRepo(t *testing.T) {
+	svc := &Service{repo: nil}
+	_, err := svc.StartRefund(100, 1000, "cancel_refund", "test")
+	if !errors.Is(err, ErrDependencyUnavailable) {
+		t.Fatalf("StartRefund() error = %v, want ErrDependencyUnavailable", err)
+	}
+}
+
+func TestServiceStartRefundWithInvalidParams(t *testing.T) {
+	svc := &Service{repo: &Repository{}}
+
+	// 测试 orderID 为 0
+	_, err := svc.StartRefund(0, 1000, "cancel_refund", "test")
+	if !errors.Is(err, ErrRefundCannotStart) {
+		t.Fatalf("StartRefund() error = %v, want ErrRefundCannotStart", err)
+	}
+
+	// 测试金额为 0
+	_, err = svc.StartRefund(100, 0, "cancel_refund", "test")
+	if !errors.Is(err, ErrRefundCannotStart) {
+		t.Fatalf("StartRefund() error = %v, want ErrRefundCannotStart", err)
+	}
+}
+
+func TestServiceWalletRechargeDisabledInProduction(t *testing.T) {
+	svc := NewService(&Repository{}, "production")
+	_, err := svc.StartWalletRecharge(1, WalletRechargePaymentRequest{AmountCent: 1000}, "127.0.0.1")
+	if !errors.Is(err, ErrWalletRechargeDisabled) {
+		t.Fatalf("StartWalletRecharge() error = %v, want ErrWalletRechargeDisabled", err)
+	}
+}
+
+func TestServiceWalletRechargeEnabledInDevelopment(t *testing.T) {
+	svc := NewService(&Repository{}, "development")
+	if svc.walletRechargeEnabled != true {
+		t.Fatal("walletRechargeEnabled should be true in development")
+	}
+}
+
+// TestRefundBizTypeConstants 测试退款业务类型常量
+func TestRefundBizTypesContainsExpectedValues(t *testing.T) {
+	expected := []string{
+		"cancel_refund",
+		"admin_close_refund",
+		"admin_refund",
+		"checkout_refund",
+		"deposit_refund",
+		"rent_refund",
+		"arbitration_refund",
+	}
+
+	for _, bizType := range expected {
+		found := false
+		for _, refundType := range refundBizTypes {
+			if refundType == bizType {
+				found = true
+				break
+			}
+		}
+		if !found {
+			t.Fatalf("refundBizTypes missing expected type: %s", bizType)
+		}
+	}
+}
+
+// TestRuntimePaymentConfigMockMode 测试 mock 模式判断
+func TestRuntimePaymentConfigIsMockMode(t *testing.T) {
+	config := runtimePaymentConfig{Provider: "mock"}
+	if !config.isMockMode() {
+		t.Fatal("isMockMode() = false, want true for mock provider")
+	}
+
+	config.Provider = "lakala"
+	if config.isMockMode() {
+		t.Fatal("isMockMode() = true, want false for lakala provider")
+	}
+}
+
+// TestChannelSourceConstants 测试渠道来源常量
+func TestChannelSourceConstantsAreDefined(t *testing.T) {
+	sources := []string{
+		channelSourceCreate,
+		channelSourceQuery,
+		channelSourceNotify,
+		channelSourceMock,
+	}
+
+	expected := []string{"create", "query", "notify", "mock"}
+
+	for i, source := range sources {
+		if source != expected[i] {
+			t.Fatalf("channelSource[%d] = %s, want %s", i, source, expected[i])
+		}
+	}
+}