LUSAKA, Zambia — Treasury Secretary Janet Yellen said the Federal Reserve likely wouldn’t accept a $1 trillion platinum coin if the Biden administration tried to mint one to avoid breaching the debt limit, dismissing an idea that has been floated to circumvent Congress on the issue. Some Biden administration officials and Democrats on Capitol Hill have discussed the possibility that the Treasury could use an obscure law authorizing platinum coins in the event of a potential default. Under the proposed scheme, the Treasury would mint a $1 trillion coin and deposit it at the Fed, and then draw the money to pay the country’s bills.