Allow overriding SlugService by resolving it from the service container

[justrau]

When instantiated using new, the SlugService cannot be overridden by app code.
Resolving SlugService from the container, allows for overriding.

[cviebrock]

Thanks for the PR @justrau. Can you look into why the tests are failing?

[justrau]

@cviebrock I've triggered Actions on my fork's new branch (https://github.com/justRau/eloquent-sluggable/commits/refs/heads/justRau-patch-2/) from master branch and looks like it is a general problem, because the same tests fail with the same messages, even though that branch does not have the code that was changed in this PR.

Tests failure on basically the master branch: https://github.com/justRau/eloquent-sluggable/actions/runs/12354157246/job/34474862720

I currently have no idea, why that error happens.

[justrau]

Also, I'm running the code from this PR in a production application using https://github.com/symplify/vendor-patches, all's ok.