NEWARK, N.J. — A cardiologist with offices in New York and New Jersey has admitted taking part in a scheme that subjected thousands of patients to unnecessary tests and treatment and resulted in $19 million in bogus bills, what authorities call the largest case of health care fraud ever by a practitioner in either state. Dr. Jose Katz pleaded guilty Wednesday in federal court to conspiracy to commit health care fraud. Katz founded Cardio-Med Services LLC and Comprehensive Healthcare & Medical Services.