已知直线\({{l}_{1}}:x-y+1=0\)与\({{l}_{2}}:x+y-1=0\)相交于点\(P\),直线\({{l}_{3}}:ax+y-a+1=0\).
\((1)\)若点\(P\)在直线\({{l}_{3}}\)上,求\(a\)的值\(;\)
\((2)\)若直线\({{l}_{3}}\)交直线\({{l}_{1}},{{l}_{2}}\)分别为点\(A\)和点\(B\),且点\(B\)的坐标为\(\left( 3,-2 \right)\),求\(\Delta PAB\)的外接圆的标准方程.