Is the marriage contract considered null and void due to the concealment of the wife's childhood neurological seizures, and what is the legitimate Shar'i solution to this issue?

The marriage contract is valid and is not invalidated by illnesses that do not reach the level of insanity. There is no obligation to disclose every defect unless it prevents enjoyment. As long as this illness does not reach the degree of insanity, it does not invalidate the contract, but it does permit the husband to annul the marriage if it prevents enjoyment.