The man and the woman arrived at the Washington steakhouse one evening in May 2019 and took a table on the patio, near where five diners already were seated and seemed to be enjoying themselves. Jackets were off, laughter was heard and wine was being poured. A cigarette dangled from one man’s hand. But there was something else that stood out: Among the five were Senator Robert Menendez of New Jersey and his future wife, Nadine Menendez. The couple who had arrived late appeared to be on a...