What is the test for establishing a cause of action for legal malpractice?

"To state a cause of action for legal malpractice, a plaintiff must plead '(1) the duty of the attorney to use such skill, prudence, and diligence as members of his or her profession commonly possess and exercise; (2) a breach of that duty; (3) a proximate causal connection between the breach and the resulting injury; and (4) actual loss or damage resulting from the attorney's negligence.' [Citation.] Whether an attorney sued for malpractice owed a duty of care to the plaintiff 'is a question of law and depends on a judicial weighing of the policy considerations for and against the imposition of liability under the circumstances.' [Citation.]" (Martorana v. Marlin & Saltzman (2009) 175 Cal.App.4th 685, 693.)

To establish causation, plaintiff must show "either (1) but for the negligence, the harm would not have occurred, or (2) the negligence was a concurrent independent cause of the harm." (Viner v. Sweet (2003) 30 Cal.4th 1232, 1241, italics omitted).) We deal here with the former. Under the "but for" test, " 'the actor's negligent conduct is not a substantial factor in bringing about harm to another if the harm would have been sustained even if the actor had not been negligent.' " (Id. at p. 1240, quoting Rest.2d Torts, 432, italics omitted.) "[T]he crucial causation inquiry is what would have

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