Is it considered unlawful (haram) to calculate overtime based on an 8-hour workday instead of the 9 hours stipulated in the contract, given my awareness that the labor law and the company's executive regulations specify working hours as 8 hours?

The decisive factor is what is stipulated in the specific employment contract between you and the company. If the contract states that working hours are nine hours, then this is what is binding, because "the contract is the law between the contracting parties," and you are not entitled to count the ninth hour as overtime.