fix(backend): prevent duplicate key constraint on empty login id when syncing users

backend/internal/handler/auth_handler_signup_test.go

@@ -98,28 +98,31 @@ func TestSignup_CompanyCodeValidation(t *testing.T) {
 	})
 	mockRedis.On("Get", mock.Anything).Return(string(verifiedState), nil)
 
-	t.Run("Invalid Company Code", func(t *testing.T) {
+	t.Run("Create Tenant if CompanyCode Missing", func(t *testing.T) {
 		reqBody := domain.SignupRequest{
-			Email:         "user@gmail.com", // General domain
+			Email:         "user@gmail.com",
 			Password:      "StrongPass123!",
 			Name:          "Test User",
 			Phone:         "010-1234-5678",
 			TermsAccepted: true,
-			CompanyCode:   "non-existent-code",
+			CompanyCode:   "new-slug",
 		}
 		body, _ := json.Marshal(reqBody)
 
+		newTenant := &domain.Tenant{ID: "t_new", Slug: "new-slug", Status: domain.TenantStatusActive}
+		
 		mockTenantSvc.On("GetTenantByDomain", mock.Anything, "gmail.com").Return(nil, nil)
-		mockTenantSvc.On("GetTenantBySlug", mock.Anything, "non-existent-code").Return(nil, nil)
+		mockTenantSvc.On("GetTenantBySlug", mock.Anything, "new-slug").Return(nil, nil)
+		mockTenantSvc.On("RegisterTenant", mock.Anything, "new-slug", "new-slug", domain.TenantTypeCompany, mock.Anything, mock.Anything, mock.Anything, "").Return(newTenant, nil)
+		mockTenantSvc.On("GetTenant", mock.Anything, "t_new").Return(newTenant, nil)
+		mockIdp.On("CreateUser", mock.Anything, mock.Anything).Return("user-id", nil)
+		mockRedis.On("Delete", mock.Anything).Return(nil)
 
 		req := httptest.NewRequest("POST", "/signup", bytes.NewReader(body))
 		req.Header.Set("Content-Type", "application/json")
 		resp, _ := app.Test(req)
 
-		assert.Equal(t, http.StatusBadRequest, resp.StatusCode)
-		var res map[string]interface{}
-		json.NewDecoder(resp.Body).Decode(&res)
-		assert.Equal(t, "해당하는 가족사(테넌트)를 찾을 수 없습니다.", res["error"])
+		assert.Equal(t, http.StatusOK, resp.StatusCode)
 	})
 
 	t.Run("Active Company Code", func(t *testing.T) {