[9.x] Fix decimal cast precision issue

[timacdonald]

Fixes #45454

Problem

Currently we are handling floats via format_number. Because of number_format, we are losing precision, if the precision is long enough.

Solution

This is a 2 part solution. This PR is part one to address this issue in the least breaking way possible for 9.x and then a follow up PR will fix this using bcmath instead.

To solve this for 9.x we are now manually handling the formatting via the Str helper.

Breaking

number_format rounds numbers when the precision is being reduced. This is no longer the case and I'm not aware of any good way to handle this.

$model = new class extends Model {
    protected $casts = [
        'amount' => 'decimal:1',
    ];
};

$model->amount = '0.99';

// The following test...
// Before ✅
// After ❌

$this->assertSame('1.0', $model->amount);

// The following test...
// Before ❌
// After ✅

$this->assertSame('0.9', $model->amount);

I'm proposing that in 10.x we change this manual string manipulation into a basic bcmath call: #45457

I feel that making bcmath a requirement to use an existing feature could be a breaking change for some applications(?)

[Sjord]

It looks like this may break on numbers containing exponents, e.g. 0.1e3. Also, should a zero be added if the int part is empty (.1)?

[timacdonald]

Follow up that improves this fix: #45492