refactor: remove Subject field from Conversation model and related logic, replace with CustomerName

internal/ai/runtime/graphs/prepare_ticket_draft_graph_test.go

@@ -9,7 +9,6 @@ import (
 
 func TestBuildPrepareTicketDraftResult_UsesConversationFallbacks(t *testing.T) {
 	conversation := models.Conversation{
-		Subject:            "企业微信登录异常",
 		LastMessageSummary: "用户反馈企业微信扫码后页面空白，无法进入工作台",
 	}
 	messages := []models.Message{
@@ -38,7 +37,6 @@ func TestBuildPrepareTicketDraftResult_UsesConversationFallbacks(t *testing.T) {
 
 func TestBuildPrepareTicketDraftResult_ReadyWithExplicitIssue(t *testing.T) {
 	conversation := models.Conversation{
-		Subject:            "订单支付失败",
 		LastMessageSummary: "用户反馈连续支付失败",
 	}