AUSTIN, Texas — Disability advocates this week threatened to sue the Texas Education Agency unless the state permanently ends its special education enrollment benchmark within the next month. The advocates said immediate action was necessary because of the “devastating harm” caused by the benchmark. The state already has suspended and pledged to eventually eliminate the decade-old benchmark, which punished school districts for giving special education services to more than 8.5 percent of students. But the state has angered advocates by not saying when it will permanently end the policy.