Revert "minor #45899 [Messenger] [Security/Core] Remove legacy class aliases (nicolas-grekas)"

This reverts commit 758ed6bf66648a17432496162cd02215b8809df4, reversing
changes made to 1bd3af4ec6303a4cf3de0218abc29ca9fdd7a793.

Author: nicolas-grekas

Authorization/TraceableAccessDecisionManager.php

@@ -109,3 +109,7 @@ public function getDecisionLog(): array
         return $this->decisionLog;
     }
 }
+
+if (!class_exists(DebugAccessDecisionManager::class, false)) {
+    class_alias(TraceableAccessDecisionManager::class, DebugAccessDecisionManager::class);
+}

Exception/UserNotFoundException.php

@@ -71,3 +71,7 @@ public function __unserialize(array $data): void
         parent::__unserialize($parentData);
     }
 }
+
+if (!class_exists(UsernameNotFoundException::class, false)) {
+    class_alias(UserNotFoundException::class, UsernameNotFoundException::class);
+}

Tests/Authorization/TraceableAccessDecisionManagerTest.php

@@ -15,6 +15,7 @@
 use Symfony\Component\Security\Core\Authentication\Token\TokenInterface;
 use Symfony\Component\Security\Core\Authorization\AccessDecisionManager;
 use Symfony\Component\Security\Core\Authorization\AccessDecisionManagerInterface;
+use Symfony\Component\Security\Core\Authorization\DebugAccessDecisionManager;
 use Symfony\Component\Security\Core\Authorization\TraceableAccessDecisionManager;
 use Symfony\Component\Security\Core\Authorization\Voter\VoterInterface;
 
@@ -170,6 +171,13 @@ public function provideObjectsAndLogs(): \Generator
         ];
     }
 
+    public function testDebugAccessDecisionManagerAliasExistsForBC()
+    {
+        $adm = new TraceableAccessDecisionManager(new AccessDecisionManager());
+
+        $this->assertInstanceOf(DebugAccessDecisionManager::class, $adm, 'For BC, TraceableAccessDecisionManager must be an instance of DebugAccessDecisionManager');
+    }
+
     /**
      * Tests decision log returned when a voter call decide method of AccessDecisionManager.
      */