fix: call markSealed before showCompleted when language server is null

Author: xinyi-gong

com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/ThinkingTurnWidget.java

@@ -82,6 +82,7 @@ public void sealThinking() {
     }
     CopilotLanguageServerConnection ls = CopilotCore.getPlugin().getCopilotLanguageServer();
     if (ls == null) {
+      target.markSealed();
       target.showCompleted(Messages.thinking_completedTitle);
       requestLayout();
       return;