[5.x] fix unique entry value validation for date fields

[simonerd]

The UniqueEntryValue validation doesn't work with date fields. The query doesn't seem to work if $value is a carbon instance:

$existing = $query
    ->when(
        is_array($value),
        fn ($query) => $query->whereIn($attribute, $value),
        fn ($query) => $query->where($attribute, $value)
    )
    ->first();

In this PR I simply add toDateString() to $value if it is a Carbon instance.
EDIT: In this PR I added a check to handle Carbon values, both for single and range values. I also added matching tests.

[simonerd]

Uh, I should cover the array case as well 🙈 Back to draft.
Maybe a test wouldn't harm either.

EDIT: mode: range handling added, plus tests ✔️