Simmering tensions between Anthem Inc. and Cigna Corp. exploded Tuesday as Cigna sued to end their $48 billion deal, and Anthem moments later said the smaller health insurer had no right to do so and that it would fight to keep the merger alive. As part of its lawsuit, Cigna is seeking a $1.85 billion breakup fee, plus $13 billion in additional damages it says are owed after “the path for regulatory approval of the transaction was fatally compromised” by Anthem.