The board is always 9 cells by 9 cells, and every cell only contains integers from 0 to 9. So, this is not within the scope of kata

Even though I see your point, why would you even try data outside the valid inputs?

Add this tests to your solution and check it:

Test.assertEquals(validSolution([[5, 5, 5, 5, 5, 5, 5, 5, 5], 
                                 [5, 5, 5, 5, 5, 5, 5, 5, 5],
                                 [5, 5, 5, 5, 5, 5, 5, 5, 5],
                                 [5, 5, 5, 5, 5, 5, 5, 5, 5],
                                 [5, 5, 5, 5, 5, 5, 5, 5, 5],
                                 [5, 5, 5, 5, 5, 5, 5, 5, 5],
                                 [5, 5, 5, 5, 5, 5, 5, 5, 5],
                                 [5, 5, 5, 5, 5, 5, 5, 5, 5],
                                 [5, 5, 5, 5, 5, 5, 5, 5, 5]]), false);
                                 
Test.assertEquals(validSolution([[45, 0, 0, 0, 0, 0, 0, 0, 0], 
                                 [0, 0, 0, 45, 0, 0, 0, 0, 0],
                                 [0, 0, 0, 0, 0, 0,45, 0, 0],
                                 [0,45, 0, 0, 0, 0, 0, 0, 0],
                                 [0, 0, 0, 0,45, 0, 0, 0, 0],
                                 [0, 0, 0, 0, 0, 0, 0,45, 0],
                                 [0, 0,45, 0, 0, 0, 0, 0, 0],
                                 [0, 0, 0, 0, 0,45, 0, 0, 0],
                                 [0, 0, 0, 0, 0, 0, 0, 0,45]]), false);