No surprise with the warranty, I expected as much!
At the end of the day the dealer is responsible under the SOGA to either repair the car FOC or accept it back and refund you in full.
If you do wish to continue with a repair according to my information P0138 is the bank 1 sensor 2 which is the post cat one on the passenger side Lexus part number 89465-50140.
You could probably buy a Denso ( which are same as the Lexus/Toyota ones) for half the Lexus price.
I would personally get another diagnosis from a trusted local independant garage as I am surprised that an O2 sensor is causing a rough idle from cold, O2 sensors don't begin operating correctly until the engine has warmed up and the post cat sensors only monitor the gases they don't control fuel ratios.