By Steve Holland and Kanishka Singh WASHINGTON (Reuters) – Senior White House adviser Amos Hochstein is expected to visit Beirut on Thursday, a U.S. official said late on Wednesday, as part of U.S. efforts to ease tensions along the Israel-Lebanon border. Washington fears Israel’s war in Gaza could spread violence across the region, with armed groups backed by Israel’s arch-rival Iran launching solidarity attacks in Lebanon, Syria, Iraq and Yemen. Lebanese armed group Hezbollah has been trading fire with the Israeli military across Lebanon’s southern border since Palestinian Islamist group Ham…