style: update chat todo widget with box-shadow and remove border (microsoft#259894)

Author: bhavyaus

src/vs/workbench/contrib/chat/browser/media/chat.css

@@ -2370,9 +2370,8 @@ have to be updated for changes to the rules above, or to support more deeply nes
 	top: 0;
 	z-index: 100;
 	background-color: var(--vscode-sideBar-background);
-	border-bottom: 1px solid var(--vscode-chat-requestBorder);
 	padding: 8px 16px;
-	margin-bottom: 8px;
+	box-shadow: 0 2px 4px var(--vscode-scrollbar-shadow);
 }
 
 .chat-todo-list-widget .todo-list-expand {