The set of all values of \( a^2 \) for which the line \( x+y=0 \) bisects two distinct chords drawn from a point \( P\!\left(\frac{1+a}{2},\frac{1-a}{2}\right) \) on the circle \( 2x^2+2y^2-(1+a)x-(1-a)y=0 \), is equal to:
\( (8,\infty) \)
\( (4,\infty) \)
\( (2,12] \)
\( (0,4] \)