fix: allow plain function name callbacks in Dispatcher::execute()

Since v3.19.2 every string callback went through parseStringClassAndMethod(),
so routes and events mapped to a global function name threw
InvalidArgumentException. Call existing functions directly and keep the
Class->method / Class::method parsing as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

flight/core/Dispatcher.php

@@ -299,6 +299,10 @@ class Dispatcher
         $this->verifyValidFunction($callback);
 
         if (is_string($callback)) {
+            if (function_exists($callback)) {
+                return $callback(...$params);
+            }
+
             $callback = $this->parseStringClassAndMethod($callback);
         }
 

tests/DispatcherTest.php

@@ -168,6 +168,11 @@ class DispatcherTest extends TestCase
         $this->dispatcher->execute('nonexistentGlobalFunction');
     }
 
+    public function testExecuteGlobalFunctionString(): void
+    {
+        $this->assertSame('HELLO', $this->dispatcher->execute('strtoupper', ['hello']));
+    }
+
     // It will be useful for executing instance Controller methods statically
     public function testCanExecuteAnNonStaticMethodStatically(): void
     {