Fix Windows test failures: proper WSAPoll error handling and FD_SETSIZE check

lib/u_async.c

@@ -438,7 +438,11 @@ err_t uasync_cancel_timeout(struct UASYNC* ua, void* t_id) {
 
 // Instance version
 void* uasync_add_socket(struct UASYNC* ua, int fd, socket_callback_t read_cbk, socket_callback_t write_cbk, socket_callback_t except_cbk, void* user_data) {
-    if (!ua || fd < 0 || fd >= FD_SETSIZE) return NULL;  // Bounds check
+    if (!ua || fd < 0) return NULL;
+    // FD_SETSIZE check only for POSIX systems - Windows sockets can have any value
+#ifndef _WIN32
+    if (fd >= FD_SETSIZE) return NULL;
+#endif
 
     int index = socket_array_add(ua->sockets, fd, read_cbk, write_cbk, except_cbk, user_data);
     if (index < 0) return NULL;