Is the questioner considered to be the son of adultery if his parents contracted their marriage with a Qadiani shaykh while secretly being Muslims, and they did not renew the contract with a Sunni shaykh until after his birth? And does that make him a Qadiani, obligating him to repent and embrace Islam?

There is no blame on the parents for concealing their faith before marriage due to fear, as Allah Almighty says: " إلا أن تتقوا منهم تقاة (unless you genuinely fear something from them)." The involvement of a Qadiani in the marriage contract does not harm if the woman's guardian is a Muslim, or if he is a non-believer and his involvement is merely administrative. For the religion of Allah is ease. The marriage of the parents is valid, as the Hanafi school of thought permits the omission of a guardian in marriage out of necessity. Furthermore, renewing the marriage contract and publicizing it resolves any issues. Their marriage establishes lineage, and you are not considered a child of fornication. You are a Muslim, following your Muslim parents.