[!!!TASK] Make all classes final

This library has a very large number of extension points via
interfaces and abstract classes. All other classes should not
be extended by users and shall theirfor be marked as final.

There are a few classes left that should also be marked as final
but those require some extra attention.

Author: jaapio

src/Config/Configuration.php

@@ -19,7 +19,7 @@
 
 use function assert;
 
-class Configuration implements ConfigurationInterface
+final class Configuration implements ConfigurationInterface
 {
     public function getConfigTreeBuilder(): TreeBuilder
     {

src/Config/XmlFileLoader.php

@@ -23,7 +23,7 @@
 use function is_string;
 use function sprintf;
 
-class XmlFileLoader extends FileLoader
+final class XmlFileLoader extends FileLoader
 {
     /** @return mixed[][] */
     public function load(mixed $resource, string|null $type = null): array

src/DependencyInjection/ApplicationExtension.php

@@ -26,7 +26,7 @@
 use function dirname;
 use function sprintf;
 
-class ApplicationExtension extends Extension implements CompilerPassInterface
+final class ApplicationExtension extends Extension implements CompilerPassInterface
 {
     /** @param string[] $configs */
     public function load(array $configs, ContainerBuilder $container): void

tests/unit/Logger/SpyProcessorTest.php

@@ -15,7 +15,7 @@
 
 use PHPUnit\Framework\TestCase;
 
-class SpyProcessorTest extends TestCase
+final class SpyProcessorTest extends TestCase
 {
     public function testHasBeenCalledReturnsFalseByDefault(): void
     {