Holding that a private insurance company, acting as fiscal intermediary or carrier on behalf of the United States in administration of a Medicare program, was entitled to official immunity from suit for claims that arise out of performance of its duty to investigate and report possible Medicare fraud and rejecting the argument that the denial of immunity in Richardson v. McKnight, 521 U.S. 399, 117 S.Ct. 2100, 138 L.Ed.2d 540, supported the denial of immunity to those private entities that act "on behalf of the [government] in carrying out certain administrative responsibilities that the law imposes"
Holding that "the public character of this park requires that it be treated as a public institution subject to the command of the Fourteenth Amendment, regardless of who now has title under state law"