GLightbox image formatters lose services when serialized

Problem/Motivation

Both image formatters declare the module-handler and library-discovery services as private. Their inherited DependencySerializationTrait cannot restore these properties after serialization. Calling settingsForm() afterward fails with Call to a member function moduleExists() on null.

Steps to reproduce

1. Create a GLightbox or GLightbox Responsive formatter through the formatter plugin manager.
2. Serialize and unserialize the formatter.
3. Call settingsForm() on the restored formatter. Both injected services are null, and the call fails.

Proposed resolution

Change the four service properties to protected, following Drupal's serialization guidance. Add a regression test covering both formatters.

Locally tested on Drupal 10 and 11: the regression test fails before the fix and passes afterward.

Remaining tasks

Review and merge the MR.