* fix: handle enum instances in ModelNotFoundException message

When passing a BackedEnum to findOrFail(), the enum gets forwarded to
ModelNotFoundException::setModel() which tries to implode the IDs into
a string. Since enums can't be implicitly cast to string, this throws
a fatal error. This converts BackedEnum instances to their underlying
value before storing them in the ids array.

* Update ModelNotFoundException.php

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Co-authored-by: Taylor Otwell <taylor@laravel.com>