feat(agents): align duplicate create errors

internal/handler/api/v1/agent_handler.go

@@ -111,7 +111,10 @@ func (h *AgentHandler) Create(c *gin.Context) {
 	agent, svcErr := h.svc.Create(c.Request.Context(), accountID, userID, req)
 	if svcErr != nil {
 		if errors.Is(svcErr, repository.ErrAlreadyMember) {
-			response.AbortWithStatusError(c, http.StatusConflict, response.ErrConflict, svcErr.Error())
+			c.JSON(http.StatusUnprocessableEntity, gin.H{
+				"message":    "User has already been taken",
+				"attributes": []string{"user_id"},
+			})
 			return
 		}
 		applogger.L().Errorf("Create agent for account %d: %v", accountID, svcErr)

internal/handler/api/v1/agent_handler_test.go

@@ -190,10 +190,14 @@ func (s *AgentHandlerTestSuite) TestCreateAgentDuplicate() {
 	s.handler.Create(c)
 	assert.Equal(s.T(), http.StatusOK, w.Code)
 
-	// Try creating again — should conflict
+	// Try creating again — Chatwoot renders ActiveRecord::RecordInvalid as 422.
 	w2, c2 := s.makeRequest("POST", "/api/v1/accounts/1/agents", req, s.account.ID, s.user.ID)
 	s.handler.Create(c2)
-	assert.Equal(s.T(), http.StatusConflict, w2.Code)
+	assert.Equal(s.T(), http.StatusUnprocessableEntity, w2.Code)
+	var data map[string]interface{}
+	json.Unmarshal(w2.Body.Bytes(), &data)
+	assert.Equal(s.T(), "User has already been taken", data["message"])
+	assert.Equal(s.T(), []interface{}{"user_id"}, data["attributes"])
 }
 
 func (s *AgentHandlerTestSuite) TestCreateAgentValidation() {