refactor: change pointer receivers to value types for Conversation and UserMessage in various structs and functions

internal/ai/runtime/graphs/triage_service_request_graph_test.go

@@ -8,7 +8,7 @@ import (
 )
 
 func TestTriageServiceRequestResult_PrepareTicket(t *testing.T) {
-	conversation := &models.Conversation{
+	conversation := models.Conversation{
 		Subject:            "支付失败需要登记工单",
 		LastMessageSummary: "用户要求建单跟进支付失败问题",
 	}
@@ -30,7 +30,7 @@ func TestTriageServiceRequestResult_PrepareTicket(t *testing.T) {
 }
 
 func TestTriageServiceRequestResult_Handoff(t *testing.T) {
-	conversation := &models.Conversation{
+	conversation := models.Conversation{
 		Subject:            "投诉转人工",
 		LastMessageSummary: "用户要求人工处理扣费投诉",
 	}