코워크·코드 장시간 실행 종료 직전 NullReference 실패를 방지하고 예외 추적을 보강한다

- ChatWindow 실행 경로에서 스트리밍 취소 토큰을 지역 변수로 고정해 마지막 라이브 프리뷰 단계에서 _streamCts null 참조가 발생하지 않도록 수정

- 최종 타이핑 프리뷰 컨테이너 준비 실패와 취소 예외를 방어적으로 처리해 장시간 작업이 마지막 UI 렌더 때문에 오류로 뒤집히지 않도록 정리

- 에이전트 실행 예외 전체를 앱 로그에 남기고 README 및 DEVELOPMENT 문서 이력을 갱신

src/AxCopilot/Views/ChatWindow.xaml.cs

@@ -5595,7 +5595,9 @@ public partial class ChatWindow : Window
         BtnStop.Visibility = Visibility.Visible;
         if (runTab == "Cowork" || runTab == "Code")
             BtnPause.Visibility = Visibility.Visible;
-        _streamCts = new CancellationTokenSource();
+        var streamCts = new CancellationTokenSource();
+        _streamCts = streamCts;
+        var streamToken = streamCts.Token;
         ForceScrollToEnd();
 
         var assistantContent = string.Empty;
@@ -5651,8 +5653,8 @@ public partial class ChatWindow : Window
                     preparedExecution,
                     (messages, token) => RunAgentLoopAsync(runTab, rememberTab, conversation, messages, token),
                     (messages, token) => _llm.SendAsync(messages.ToList(), token),
-                    _streamCts.Token);
-                assistantContent = response;
+                    streamToken);
+                assistantContent = response ?? string.Empty;
             }
 
             responseElapsedMs = Math.Max(0, (long)(DateTime.UtcNow - _streamStartTime).TotalMilliseconds);
@@ -5681,7 +5683,7 @@ public partial class ChatWindow : Window
             }
             else if (preparedExecution.Mode.UseAgentLoop && !string.IsNullOrWhiteSpace(assistantContent))
             {
-                await ShowTypedAssistantPreviewAsync(assistantContent, _streamCts.Token);
+                await ShowTypedAssistantPreviewAsync(assistantContent, streamToken);
             }
         }
         catch (OperationCanceledException)
@@ -5695,6 +5697,7 @@ public partial class ChatWindow : Window
         }
         catch (Exception ex)
         {
+            Services.LogService.Debug($"에이전트 실행 실패: {ex}");
             var finalized = _chatEngine.FinalizeExecutionContentForUi(assistantContent, ex);
             assistantContent = finalized.Content;
             draftFailure = finalized.FailureReason;
@@ -5734,6 +5737,9 @@ public partial class ChatWindow : Window
             return;
 
         var container = CreateStreamingContainer(out var streamText);
+        if (container == null || streamText == null || MessagePanel == null)
+            return;
+
         _activeStreamText = streamText;
         _cachedStreamContent = finalContent;
         _displayedLength = 0;
@@ -5745,8 +5751,15 @@ public partial class ChatWindow : Window
         streamText.Text = _cursorVisible ? "\u258c" : " ";
 
         var deadline = DateTime.UtcNow.AddMilliseconds(Math.Clamp(finalContent.Length * 6, 500, 1800));
-        while (_displayedLength < _cachedStreamContent.Length && DateTime.UtcNow < deadline && !ct.IsCancellationRequested)
-            await Task.Delay(30, ct);
+        try
+        {
+            while (_displayedLength < _cachedStreamContent.Length && DateTime.UtcNow < deadline && !ct.IsCancellationRequested)
+                await Task.Delay(30, ct);
+        }
+        catch (OperationCanceledException)
+        {
+            return;
+        }
 
         _displayedLength = _cachedStreamContent.Length;
         if (_activeStreamText != null)