The massive legal action initiated by the Federal Housing Finance Agency last year on behalf of Fannie Mae and Freddie Mac against many of the nation’s biggest lenders is getting ready to face its first legal challenge, and the federal judge’s ruling will determine the scope and direction of the cases, experts say. The FHFA lawsuits seek tens of billions of dollars in damages for losses incurred by Fannie and Freddie on purchases of approximately $200 billion in residential mortgage-backed securities.