fix: 移除 HttpClient 中的 curl_close() 调用

PHP 8.0+ 中 curl 资源会自动关闭，curl_close() 已被弃用。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/util/HttpClient.php

@@ -47,7 +47,7 @@ class HttpClient
         $httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
         $error = curl_error($ch);
 
-        curl_close($ch);
+        // PHP 8.0+ curl 资源自动关闭，无需 curl_close()
 
         self::logRequest('GET', $url, [], $response, $httpCode);
 
@@ -103,7 +103,7 @@ class HttpClient
         $httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
         $error = curl_error($ch);
 
-        curl_close($ch);
+        // PHP 8.0+ curl 资源自动关闭，无需 curl_close()
 
         self::logRequest('POST', $url, $data, $response, $httpCode);