Reference solution used for random tests is clearly incorrect:

TriangleFitsInSquare(415339.9863145371, 459659.20499247574, 666405.0066220658, 479755.37244198116)

Test Failed
  Expected: False
  But was:  True

At a = 415339.9863145371 pointing 88 degrees clockwise and b = 459659.20499247574 pointing ~172.43 degrees counter-clockwise, the third side equals c = 666405.0066220658 and the bounding square is around 471795.42395730014, smaller than 479755.37244198116.

You can assume that the sides represent always a valid triangle.

One fixed test case is TriangleFitsInSquare(120.378, 340.873, 220.494, 600.698), which violates the triangle equality (and expects true for some reasons).

There are also cases like TriangleFitsInSquare(101.419, 184.952, 83.533, 454.56) which is a triangle with 0 area.