Texas Government Code - GOV'T § 1202.006. Validity and Incontestability

(a) A public security and any contract the proceeds of which are pledged to the payment of the public security are valid and incontestable in a court or other forum and are binding obligations for all purposes according to their terms:

(b) In any action brought to enforce the collection of county or municipal bonds that are payable from ad valorem taxes and that have been approved by the attorney general and registered by the comptroller, the certificate of the attorney general shall be admitted as evidence of the validity of the bonds and the interest coupons pertaining to the bonds.