How can we ensure that when creating a contact related to an account, the contact owner is set to be the same as the lead owner?

If a Contact record is being created manually by a User the Contact Owner can't be edited during Create, it will default to the User who is creating the Contact.  The user who creates the Contact is not the Account Owner they would need to manually change the Contact Owner AFTER Create/Save.

So you'll either need a Flow that auto updates the Contact Owner field like @Nikhil M C suggested, or a Validation Rule that blocks Users from creating new Contact records in Accounts that they do not own.