fix: store SPL closures in register() so unregister() can remove them

[michalsn]

Description
Fixed a bug where Autoloader::unregister() could fail to remove handlers from the SPL autoload stack during tests. First-class callable syntax ($this->loadClass(...)) creates a new Closure object on each call, so the callbacks passed to spl_autoload_unregister() did not match the ones originally registered with spl_autoload_register(). As a result, autoload handlers accumulated on the SPL stack instead of being removed.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide