When auto executives learned last year that President Trump’s punishing tariffs on foreign car parts were set to cost their companies billions of dollars, they were rattled. Eager to lessen the blow, they began calling and visiting Commerce Secretary Howard Lutnick in his spacious, wood-paneled office near the White House, where they negotiated, complained and cajoled in talks that stretched on for months. Then, in October, there came a breakthrough: Mr. Lutnick and his aides informed the