The Consumer Product Safety Commission said Thursday that a leading discount department store had agreed to pay a $3.9 million fine for failing to report that it continued to sell defective children’s clothing after the agency warned that the clothes could cause injuries or strangulation deaths. According to the settlement, Ross Stores knowingly failed to report to the commission within 24 hours that it had sold or kept in its stores from January 2009 to February 2012 about 23,000 children’s jackets and sweatshirts that had drawstrings at the neck or around the waist.