chore(addon/aikefu): 优化最后关闭sse的处理，刷新输出

src/addon/aikefu/api/controller/Kefu.php

@@ -749,6 +749,11 @@ class Kefu extends BaseApi
                 'content' => $assistant_content, 
             ];
             echo "event: done\ndata: " . json_encode($done_data) . "\n\n";
+            // 只在有输出缓冲时才刷新
+            if (ob_get_level() > 0) {
+                ob_flush();
+            }
+            flush();            
             
             // 发送连接关闭事件，让客户端知道连接已正常结束
             echo "event: close\ndata: {\"status\":\"completed\"}\n\n";