[9.x] Fix inconsistent results of firstOrNew() when using withCasts()

[Attia-Ahmed]

The issue is when using withCasts to override model casts, leads to inconsistent results when using firstOrNew method or any similar method depends on Model::newInstance() such as firstOrCreate(), updateOrCreate. the results are differ depends on whether the element exists on DB or not, as one result of Model $casts and other form withCasts().

For example:
If we have a Model with $casts = ['time' => 'datetime'];
and by running these queries:

$result1 = Model::query()->withCasts(['time' => 'string'])
    ->firstOrCreate(['time' => '07:30']);

$result2 =Model::query()->withCasts(['time' => 'string'])
    ->firstOrCreate(['time' => '07:30']);

both queries will leads to different entries on database as under the hood where method is looking for time = 07:30 and fail to find one, then newInstance() method will create new instance with datetime cast, so the next query will fail to find time = 07:30 and this behavior will remain for every use of firstOrCreate unless there is an existent entire in DB matches the attribute.

So developer won't be able to depend on firstOrCreate, updateOrCreate or firstOrNew to have consistent and unique data in the database in such cases.

I have started with failing tests to prove the issue, and end up with solving it by modifing newInstance method to fill model attributes after mergeCasts.