refactor: remove ExternalSource and ExternalID from Conversation model and related services, update references to use ChannelID instead

internal/pkg/dto/request/conversation_request.go

@@ -12,7 +12,6 @@ const (
 
 type ConversationListRequest struct {
 	Status            int    `json:"status"`
-	ExternalSource    string `json:"externalSource"`
 	ServiceMode       int    `json:"serviceMode"`
 	CurrentAssigneeID int64  `json:"currentAssigneeId"`
 	Keyword           string `json:"keyword"`

internal/pkg/dto/response/conversation_response.go

@@ -22,8 +22,6 @@ type ConversationResponse struct {
 	AIAgentID                 int64                           `json:"aiAgentId"`
 	ChannelID                 int64                           `json:"channelId"`
 	CustomerID                int64                           `json:"customerId"`
-	ExternalSource            enums.ExternalSource            `json:"externalSource"`
-	ExternalID                string                          `json:"externalId"`
 	Subject                   string                          `json:"subject"`
 	Status                    enums.IMConversationStatus      `json:"status"`
 	ServiceMode               enums.IMConversationServiceMode `json:"serviceMode"`