Let \( M \) and \( m \) respectively be the maximum and minimum values of the function \( f(x)=\tan^{-1}(\sin x+\cos x) \) in \( \left[0,\frac{\pi}{2}\right] \). Then the value of \( \tan(M-m) \) is equal to:
\( 3-2\sqrt{2} \)
\( 2-\sqrt{3} \)
\( 3+2\sqrt{2} \)
\( 2+\sqrt{3} \)