test: 为测试 stub 添加缺失的 GroupRepository 接口方法

新增 BindAccountsToGroup 和 GetAccountIDsByGroupIDs 方法的 stub 实现，
确保测试文件中的 mock 类型满足 GroupRepository 接口要求。

backend/internal/service/admin_service_delete_test.go

@@ -164,6 +164,14 @@ func (s *groupRepoStub) DeleteAccountGroupsByGroupID(ctx context.Context, groupI
 	panic("unexpected DeleteAccountGroupsByGroupID call")
 }
 
+func (s *groupRepoStub) BindAccountsToGroup(ctx context.Context, groupID int64, accountIDs []int64) error {
+	panic("unexpected BindAccountsToGroup call")
+}
+
+func (s *groupRepoStub) GetAccountIDsByGroupIDs(ctx context.Context, groupIDs []int64) ([]int64, error) {
+	panic("unexpected GetAccountIDsByGroupIDs call")
+}
+
 type proxyRepoStub struct {
 	deleteErr    error
 	countErr     error

backend/internal/service/admin_service_group_test.go

@@ -108,6 +108,14 @@ func (s *groupRepoStubForAdmin) DeleteAccountGroupsByGroupID(_ context.Context,
 	panic("unexpected DeleteAccountGroupsByGroupID call")
 }
 
+func (s *groupRepoStubForAdmin) BindAccountsToGroup(_ context.Context, _ int64, _ []int64) error {
+	panic("unexpected BindAccountsToGroup call")
+}
+
+func (s *groupRepoStubForAdmin) GetAccountIDsByGroupIDs(_ context.Context, _ []int64) ([]int64, error) {
+	panic("unexpected GetAccountIDsByGroupIDs call")
+}
+
 // TestAdminService_CreateGroup_WithImagePricing 测试创建分组时 ImagePrice 字段正确传递
 func TestAdminService_CreateGroup_WithImagePricing(t *testing.T) {
 	repo := &groupRepoStubForAdmin{}
@@ -378,3 +386,11 @@ func (s *groupRepoStubForFallbackCycle) GetAccountCount(_ context.Context, _ int
 func (s *groupRepoStubForFallbackCycle) DeleteAccountGroupsByGroupID(_ context.Context, _ int64) (int64, error) {
 	panic("unexpected DeleteAccountGroupsByGroupID call")
 }
+
+func (s *groupRepoStubForFallbackCycle) BindAccountsToGroup(_ context.Context, _ int64, _ []int64) error {
+	panic("unexpected BindAccountsToGroup call")
+}
+
+func (s *groupRepoStubForFallbackCycle) GetAccountIDsByGroupIDs(_ context.Context, _ []int64) ([]int64, error) {
+	panic("unexpected GetAccountIDsByGroupIDs call")
+}

backend/internal/service/gateway_multiplatform_test.go

@@ -266,6 +266,14 @@ func (m *mockGroupRepoForGateway) DeleteAccountGroupsByGroupID(ctx context.Conte
 	return 0, nil
 }
 
+func (m *mockGroupRepoForGateway) BindAccountsToGroup(ctx context.Context, groupID int64, accountIDs []int64) error {
+	return nil
+}
+
+func (m *mockGroupRepoForGateway) GetAccountIDsByGroupIDs(ctx context.Context, groupIDs []int64) ([]int64, error) {
+	return nil, nil
+}
+
 func ptr[T any](v T) *T {
 	return &v
 }

backend/internal/service/gemini_multiplatform_test.go

@@ -218,6 +218,14 @@ func (m *mockGroupRepoForGemini) DeleteAccountGroupsByGroupID(ctx context.Contex
 	return 0, nil
 }
 
+func (m *mockGroupRepoForGemini) BindAccountsToGroup(ctx context.Context, groupID int64, accountIDs []int64) error {
+	return nil
+}
+
+func (m *mockGroupRepoForGemini) GetAccountIDsByGroupIDs(ctx context.Context, groupIDs []int64) ([]int64, error) {
+	return nil, nil
+}
+
 var _ GroupRepository = (*mockGroupRepoForGemini)(nil)
 
 // mockGatewayCacheForGemini Gemini 测试用的 cache mock