AX Agent timeline 조립 helper를 분리해 메시지 렌더 구조를 단순화한다

RenderMessages()가 직접 처리하던 visible 메시지/이벤트 필터링과 timestamp/order 기반 timeline action 조립을 ChatWindow.TimelinePresentation.cs로 이동해 메인 렌더 루프가 orchestration 중심으로 더 단순해지도록 정리했다.

README와 DEVELOPMENT 문서에 2026-04-06 10:36 (KST) 기준 이력을 반영했고, dotnet build 검증 결과 경고 0 / 오류 0을 확인했다.

src/AxCopilot/Views/ChatWindow.xaml.cs

@@ -2723,20 +2723,8 @@ public partial class ChatWindow : Window
         lock (_convLock) conv = _currentConversation;
         _appState.RestoreAgentRunHistory(conv?.AgentRunHistory);
 
-        var visibleMessages = conv?.Messages?.Where(msg =>
-        {
-            if (string.Equals(msg.Role, "system", StringComparison.OrdinalIgnoreCase))
-                return false;
-
-            if (string.Equals(msg.Role, "assistant", StringComparison.OrdinalIgnoreCase)
-                && string.IsNullOrWhiteSpace(msg.Content))
-                return false;
-
-            return true;
-        }).ToList() ?? new List<ChatMessage>();
-        var visibleEvents = (conv?.ShowExecutionHistory ?? true)
-            ? conv?.ExecutionEvents?.ToList() ?? new List<ChatExecutionEvent>()
-            : new List<ChatExecutionEvent>();
+        var visibleMessages = GetVisibleTimelineMessages(conv);
+        var visibleEvents = GetVisibleTimelineEvents(conv);
 
         if (conv == null || (visibleMessages.Count == 0 && visibleEvents.Count == 0))
         {
@@ -2752,17 +2740,7 @@ public partial class ChatWindow : Window
 
         EmptyState.Visibility = Visibility.Collapsed;
 
-        var timeline = new List<(DateTime Timestamp, int Order, Action Render)>();
-        foreach (var msg in visibleMessages)
-            timeline.Add((msg.Timestamp, 0, () => AddMessageBubble(msg.Role, msg.Content, animate: false, message: msg)));
-
-        foreach (var executionEvent in visibleEvents)
-        {
-            var restoredEvent = ToAgentEvent(executionEvent);
-            timeline.Add((executionEvent.Timestamp, 1, () => AddAgentEventBanner(restoredEvent)));
-        }
-
-        var orderedTimeline = timeline.OrderBy(x => x.Timestamp).ThenBy(x => x.Order).ToList();
+        var orderedTimeline = BuildTimelineRenderActions(visibleMessages, visibleEvents);
         var hiddenCount = Math.Max(0, orderedTimeline.Count - _timelineRenderLimit);
         if (hiddenCount > 0)
             MessagePanel.Children.Add(CreateTimelineLoadMoreCard(hiddenCount));