You don't have to prove on your tax return that you had Medicaid. If you know that you were covered by Medicaid all year, just tell TurboTax that you had health insurance coverage all year. You don't have to say what kind of coverage it was, as long as it wasn't from the Marketplace (Obamacare).
You should have received a form from the state showing that you had Medicaid coverage. I think it's Form 1095-B. But you don't need the form to file your tax return.