Let \( f(x)=x+\dfrac{a}{\pi^2-4}\sin x+\dfrac{b}{\pi^2-4}\cos x, \; x\in \mathbb{R} \) be a function which satisfies\( f(x)=x+\displaystyle \int_{0}^{\pi/2}\sin(x+y)f(y)\,dy \). Then \( (a+b) \) is equal to:
\( -\pi(\pi-2) \)
\( -2\pi(\pi-2) \)
\( -2\pi(\pi+2) \)
\( -\pi(\pi+2) \)