In determining award, about the only way you have a protest sustained is if you deviate from your stated evaluation criteria.  Here is the FAR cite 

52.217-5 Evaluation of Options.

As prescribed in 17.208(c), insert a provision substantially the same as the following:

Evaluation of Options (July 1990)

Except when it is determined in accordance with FAR 17.206(b) not to be in the Government’s best interests, the Government will evaluate offers for award purposes by adding the total price for all options to the total price for the basic requirement. Evaluation of options will not obligate the Government to exercise the option(s).

The way i would address this in the evalaution criteria is to explicitely say the contract price will be evaluated on the basis of the base year and the four prepriced option years.  That specifically excludes the up to 6 month bridge option that is causing concern.

I have never included this up to six month extension as part of an evaluation criteria and do not know of any cases where a contractor protested based on not including this.  bottom line, you are are pretty solid ground to not include it unless for some reason it was explicitly included in the evaluation criteria.