Mathematics Question from JEE Main 2025 · 4 April, Shift 1

Let $A = \begin{bmatrix} \cos \theta & 0 & -\sin \theta\\ 0 & 1 & 0\\ \sin \theta & 0 & \cos \theta \end{bmatrix}$. If for some $\theta \in (0, \pi)$, $A^2 = A^T$, then the sum of the diagonal elements of the matrix $((A + I)^3 + (A - I)^3 - 6A)$ is equal to _____.