Complete Attack on RLWE Key Exchange with reused keys, without signal leakage
There is a condition that is not considered, that is, the value after modulo 257 should be in $\mathbb Z_q$. 
When $q = 257,    \mathbb Z_q = \{ -128, ... , 128 \}$, so,  $(4+128)\mod 257$ should be $-125$ rather than $132$ .
And $-125 \mod 2 = 1$.
Thus, $sk_a \neq sk_b$  and the output of  oracle $\mathcal B$ is $0$.