By David Shepardson WASHINGTON (Reuters) -U.S. Trade Representative (USTR) Katherine Tai said on Friday the vote by General Motors workers at a pickup truck plant in Mexico to approve a new contract “demonstrates the significant benefits of true collective bargaining.” This month, Reuters reported GM agreed to an 8.5% wage hike with a new, independent union at its plant in the central Mexican city of Silao. Tai said that under labor protections of the USMCA trade deal, “workers no longer have to tolerate contracts negotiated behind their backs and have the right to vote on an agreement after i…