By Steve Holland and Dan Williams JERUSALEM (Reuters) -U.S. President Joe Biden and Israeli Prime Minister Yair Lapid signed a joint pledge on Thursday to deny Iran nuclear arms, a show of unity by allies long divided over diplomacy with Tehran. The undertaking, part of a “Jerusalem Declaration” crowning Biden’s first visit to Israel as president, came a day after he told a local TV station that he was open to “last resort” use of force against Iran – an apparent move toward accommodating Israel’s calls for a “credible military threat” by world powers. “We will not allow Iran to acquire a nucl…