Ukrainian drones on Wednesday attacked St. Petersburg, Russia’s second-largest city, on the opening day of an annual economic conference that President Vladimir V. Putin has tried to cultivate into a showcase of a modern and prosperous country. It was the second time in a month that Ukraine had struck a major Russian city before an event important to Mr. Putin, as Kyiv expands a campaign of long-range strikes aimed at inflicting economic damage on Russia and demonstrating its vulnerability to attacks. In early May, Ukrainian forces hit sites in Moscow, including a high-rise apartment building near the city center, days...