An investigation by the New York State attorney general has concluded that Gov. Andrew M. Cuomo’s administration undercounted coronavirus-related deaths at nursing homes by as much as 50 percent. The count of deaths in the state’s nursing homes has been a source of controversy for Mr. Cuomo and state Health Department officials, who have been sensitive to any suggestion that decisions made at the outset of the pandemic may have caused some of those deaths, which the state puts at more than 8,700.