[9.x] Ensure decimal rule handles large values

[timacdonald]

The validator does not currently handle arbitrary length floats when comparing the size of decimal numbers.

The following code currently passes when it should not...

Validator::validate([
    'foo' => '1.88888888888888888888'
], [
    'foo' => [
        'decimal:20',
        'min:1.88888888888888888889',
    ],
]);

// [
//    "foo" => "1.88888888888888888888",
//  ]

This PR leans on brick/math further whenever we are comparing a value's size.

[timacdonald]

This was missed when fixing the decimal model cast.

[timacdonald]

Tightening this return type, as I believe it can only return these types.