In its most positive assessment of Iran in two years, the International Atomic Energy Agency reported yesterday that it had found no evidence the nation had a nuclear weapons program and that Tehran's recent cooperation with the agency has been very good. The U.N. nuclear watchdog's report, along with Europe's acceptance of a wide-ranging nuclear agreement with Tehran, capped a pivotal day for the Islamic republic's relations with the West and left little chance for the Bush administration's Iran strategy to succeed in the near term.