For \( 0 < a < 1, \) the value of the integral \( \displaystyle\int_{0}^{\pi} \frac{dx}{1 - 2a \cos x + a^2}\) is:
\( \displaystyle\frac{\pi^{2}}{\pi+a^{2}} \)
\( \displaystyle\frac{\pi}{1+a^{2}} \)
\( \displaystyle\frac{\pi^{2}}{\pi-a^{2}} \)
\(\displaystyle \frac{\pi}{1-a^{2}} \)