A walkout would be another blow to Boeing’s reputation and finances in a year marked by problems in its airplane, defense, and space operations. Boeing is preparing to learn Thursday whether 33,000 aircraft assembly workers, most of them in the Seattle area, are going on strike and shutting down production of the company’s best-selling planes.Members of the International Association of Machinists and Aerospace Workers plan to vote on whether to approve a contract offer that includes 25% pay raises over four years.