AUSTIN, Texas — A federal judge in Texas who previously ruled to dismantle the Affordable Care Act struck down a narrower but key part of the nation’s health law Thursday that requires most insurers to cover preventive services that include screenings for cancer, diabetes and mental health.Other no-cost services, including HIV screenings, are also impacted under the ruling by U.S. District Judge Reed O’Connor that opponents say will jeopardize preventive care for millions of Americans.Experts cautioned …