Remove reflection in AutoConfigurationChecker#getConfigurations

[GaetanoCerciello]

Overview

This PR removes the reflection-based approach in AutoConfigurationChecker#getConfigurations()
and replaces it with ArchUnit's public API, resolving the TODO comment that flagged this
fragile implementation.

Problem

The original implementation used Spring's ReflectionUtils to access a private method
JavaClasses.of(), which is not part of ArchUnit's public API and is brittle:

Method method = ReflectionUtils.findMethod(JavaClasses.class, "of", Iterable.class);
ReflectionUtils.makeAccessible(method);
return (JavaClasses) ReflectionUtils.invokeMethod(method, null, this.classes.values());

Solution
Uses ArchUnit's public DescribedPredicate API with the .that() method to create
a filtered view of the original JavaClasses containing only configuration classes:

DescribedPredicate<JavaClass> inConfigurations = DescribedPredicate
        .describe("is in configurations", c -> this.classes.containsKey(c.getName()));
return this.allClasses.that(inConfigurations);

Changes Made
Removed imports: java.lang.reflect.Method and org.springframework.util.ReflectionUtils
Added field: private final JavaClasses allClasses to AutoConfigurations class
Added constructor: AutoConfigurations(JavaClasses allClasses) to accept the full JavaClasses
Modified: check() method now passes javaClasses to the constructor
Refactored: getConfigurations() to use public ArchUnit API instead of reflection