Make role model sync atomic

[amrachraf6699]

Summary

Makes Role::syncModels() atomic so a failed attachment cannot remove existing role-to-model assignments.

Problem

syncModels() deleted all existing pivot records before attaching the replacement models.

When an unsaved model was supplied, the pivot insert failed because the model key was null. The exception was thrown, but the prior assignments had already been deleted.

Fix

Wrap the delete-and-attach sequence in a database transaction.

The original QueryException still propagates, but the database rolls back the pivot deletion when attachment fails.

Test

Added a regression test covering an unsaved target model:

expect(fn () => $role->syncModels($unsavedUser))
    ->toThrow(QueryException::class);

expect($assignedUser->fresh()->hasRole($role))
    ->toBeTrue();

vendor/bin/pest tests/Traits/HasAssignedModelsTest.php --filter="does not detach models when syncing an unsaved model"

Passed.