"[T]his ability to use your own money as you see fit is what constitutes ownership and ownership has to mean something; it can't be the case that the excise tax rules lump HSAs into the same category as every other product, because HSAs are the only product where someone other than the employer owns a portion of the dollars being counted.... [T]he IRS has the necessary discretionary authority to characterize employee contributions to the employee's HSA, even if facilitated by the employer through Section 106 authority, as 'excludable' without also being included in the definition of 'applicable coverage' precisely because employers don't own any of the money in an employee's HSA."