修复：截图回调禁用系统代理，解决502回调失败

requests.post回调WVP时被本地代理(127.0.0.1:7897)拦截导致502，
添加proxies=None绕过系统代理直连WVP。

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

core/screenshot_handler.py

@@ -326,7 +326,8 @@ class ScreenshotHandler:
         if callback_url:
             try:
                 url = callback_url.rstrip("/") + SNAP_CALLBACK_PATH
-                resp = requests.post(url, json=body, timeout=SNAP_CALLBACK_TIMEOUT)
+                resp = requests.post(url, json=body, timeout=SNAP_CALLBACK_TIMEOUT,
+                                     proxies={"http": None, "https": None})
                 if resp.status_code < 300:
                     logger.info("[截图] HTTP 回调成功: request_id=%s", request_id)
                     return