PARIS - The nightmare scenario goes like this: Paris police spot two suspects in a subway station. One smashes a container of deadly sarin gas on the platform as a train pulls in. The imaginary attack is at the center of an anti-terrorism exercise running until dawn Thursday to test French security and rescue services' readiness for the real thing. The "assault" targets the Invalides Metro and suburban train station on the Left Bank of Paris, close to Les Invalides, a gold-domed former hospital that houses Napoleon Bonaparte's tomb. About 500 police and firefighters wearing chemical suits will evacuate and...