Willdan Group is buying Mantis Innovation, a Houston energy services firm, for $285 million in cash. It’s a big step for Willdan, an engineering and energy efficiency consultancy that mostly works for utilities and public agencies. Mantis gives it a much stronger foothold with private commercial and industrial customers, data centers among them. Mantis employs more than 220 people and expects about $120 million in net revenue this year, with adjusted EBITDA margins in the low 20s.