KYIV — The cluttered car carrying a mother and her 12-year-old daughter seemed barely worth the attention of Russian security officials as it approached a border checkpoint. But the least conspicuous piece of luggage — a crate for a cat — was part of an elaborate, lethal plot. Ukrainian operatives had installed a hidden compartment in the pet carrier, according to security officials with knowledge of the operation, and used it to conceal components of a bomb.