What is the legal test for a plaintiff to bring an action for attorney malpractice?

Pursuant to section 340.6(a), an attorney malpractice action "shall be commenced within one year after the plaintiff discovers, or through the use of reasonable diligence should have discovered, the facts constituting the wrongful act or omission, or four years from the date of the wrongful act or omission, whichever occurs first." "[U]nder the provisions of section 340.6, discovery of the negligent act or omission initiates the [one-year] statutory period...." (Adams v. Paul (1995) 11 Cal.4th 583, 589, fn. 2, 46 Cal.Rptr.2d 594, 904 P.2d 1205.)