Do crew rest seats count towards the maximum allowed number of seats per flight attendant?
The applicable regulation, CFR121.391, says this:   


  (4) For airplanes having a seating capacity of more than 100 passengers—two flight attendants plus one additional flight attendant for each unit (or part of a unit) of 50 passenger seats above a seating capacity of 100 passengers. 


Note that the regulation only refers to seating capacity, not whether or not the occupants are "passengers" under the legal definition (fare paying person), or airline employees.  With 152 seats, that airplane has 2 seats, a part unit, of the next "50 seat unit" above 150 seats. So I would say your concern is correct and you need an extra FA to be compliant with 121.391, or at least I would assume so until the airline is able to show that the seats are not counted as "passenger capacity" due to some language in its operating certificate or some other document.  Probably something for your union's legal rep to confirm.