[Configuration] Add string validation

Author: hlecorche

src/DependencyInjection/Configuration.php

@@ -37,8 +37,18 @@ public function getConfigTreeBuilder(): TreeBuilder
 
         $rootNode
             ->children()
-                ->scalarNode('theme')->isRequired()->end()
-                ->scalarNode('icon_theme')->isRequired()->end()
+                ->scalarNode('theme')->isRequired() // @legacy SF < 7.2 (string node introduced in Symfony 7.2)
+                    ->validate()
+                        ->ifTrue(static fn (mixed $value) => !\is_string($value))
+                        ->thenInvalid('Invalid theme')
+                    ->end()
+                ->end()
+                ->scalarNode('icon_theme')->isRequired() // @legacy SF < 7.2 (string node introduced in Symfony 7.2)
+                    ->validate()
+                        ->ifTrue(static fn (mixed $value) => !\is_string($value))
+                        ->thenInvalid('icon_theme')
+                    ->end()
+                ->end()
                 ->arrayNode('twig_functions_configuration')
                     ->treatNullLike([])
                     ->prototype('variable')->end()