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

internal/ai/runtime/graphs/analyze_conversation_graph_test.go

@@ -8,7 +8,7 @@ import (
 )
 
 func TestBuildAnalyzeConversationResult_RecommendsHandoffForComplaint(t *testing.T) {
-	conversation := &models.Conversation{
+	conversation := models.Conversation{
 		Subject:            "用户投诉扣费异常",
 		LastMessageSummary: "用户反馈被重复扣费，并要求人工处理",
 	}
@@ -32,7 +32,7 @@ func TestBuildAnalyzeConversationResult_RecommendsHandoffForComplaint(t *testing
 }
 
 func TestBuildAnalyzeConversationResult_RecommendsPrepareTicket(t *testing.T) {
-	conversation := &models.Conversation{
+	conversation := models.Conversation{
 		Subject:            "订单无法支付",
 		LastMessageSummary: "用户要求登记问题并尽快处理",
 	}