LONDON — A key U.K.-EU summit planned for this summer has been thrown into jeopardy over uncertainty about how many member countries could object to granting British applicants visas under a flagship youth experience scheme. The project is at the heart of a push to re-connect Britain to continental Europe 10 years after Brexit. But two U.K officials close to the negotiations, granted anonymity to speak openly about talks between London and Brussels, told POLITICO that some EU nations are