feat(domain): 添加 SettlementService.GetBillingSummary 接口方法

问题：settlementService.GetBillingSummary 方法存在但未在接口中暴露
解决：
1. 在 SettlementService 接口添加 GetBillingSummary 方法
2. 添加 TestSettlementService_GetBillingSummary 测试

Coverage:
- GetBillingSummary: 0% -> 100%
- Total domain coverage: 72.0% -> 72.3%

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

supply-api/internal/domain/settlement.go

@@ -86,6 +86,7 @@ type SettlementService interface {
 	Cancel(ctx context.Context, supplierID, settlementID int64) (*Settlement, error)
 	GetByID(ctx context.Context, supplierID, settlementID int64) (*Settlement, error)
 	List(ctx context.Context, supplierID int64) ([]*Settlement, error)
+	GetBillingSummary(ctx context.Context, supplierID int64, startDate, endDate string) (*BillingSummary, error)
 }
 
 // 收益服务接口

supply-api/internal/domain/settlement_test.go

@@ -605,3 +605,19 @@ func TestGenerateSettlementNo(t *testing.T) {
 	// 格式为时间戳 20060102150405
 	assert.Equal(t, 14, len(no))
 }
+
+// TestSettlementService_GetBillingSummary 测试通过结算服务获取账单摘要
+func TestSettlementService_GetBillingSummary(t *testing.T) {
+	store := newMockSettlementStore()
+	earningStore := newMockEarningStore()
+	auditStore := &mockAuditStoreForSettlement{}
+
+	svc := NewSettlementService(store, earningStore, auditStore)
+
+	summary, err := svc.GetBillingSummary(context.Background(), 1001, "2024-01-01", "2024-01-31")
+	assert.NoError(t, err)
+	assert.NotNil(t, summary)
+	assert.Equal(t, "2024-01-01", summary.Period.Start)
+	assert.Equal(t, "2024-01-31", summary.Period.End)
+	assert.Equal(t, float64(1000), summary.Summary.TotalRevenue)
+}