WASHINGTON — President Donald Trump is preparing to tell the world whether he plans to follow through on his threat to pull out of the landmark nuclear accord with Iran and almost surely ensure its collapse. There are no signs that European allies enlisted to “fix” the deal have persuaded him to preserve it. In a burst of last-minute diplomacy, punctuated by a visit by Britain’s top diplomat, the deal’s European members gave in to many of Trump’s demands, according to officials, diplomats and others briefed on the negotiations.