"[T]he justices announced on [December 22] that they will hear oral arguments on Jan. 7 on two federal policies: a vaccine-or-test mandate for workers at large employers, and a vaccine mandate for health care workers at facilities that receive federal funding.... [T]he formal question in both disputes is whether the government should be allowed to enforce the policies while litigation challenging them continues. But the justices' views on whether to grant emergency relief will likely be influenced by their views on the merits of the underlying challenges themselves."