fix: exclude timeout from scan failure rate calculation (#578)

Timeout is a normal scan result when firewalls drop packets, not a
scan failure. Only resource exhaustion errors count toward failure rate.

core/port_scan.go

@@ -400,17 +400,13 @@ func scanSinglePort(ctx context.Context, host string, port int, addr string, tim
 }
 
 // handleConnectionFailure 处理连接失败
+// 只收集资源耗尽类错误，timeout 是正常的扫描结果（防火墙 drop）不计入失败
 func handleConnectionFailure(err error, host string, port int, addr string, failedCollector *failedPortCollector) {
-	if isResourceExhaustedError(err) || isTimeoutError(err) {
+	if isResourceExhaustedError(err) {
 		failedCollector.Add(host, port, addr)
 	}
 }
 
-// isTimeoutError 判断是否为超时错误
-func isTimeoutError(err error) bool {
-	return err != nil && strings.Contains(err.Error(), "i/o timeout")
-}
-
 // verifyProxyConnectionDeep 深度验证代理连接是否真正可用
 // 防止透明代理/全回显代理的假连接问题
 // 返回: (是否有效, 验证方式)