fix: correct error handler callback signature in IntegrationValidator

Updated the dummy error handler in `checkHandlers()` to use a fully typed
callable matching PHP's expected signature for `set_error_handler()`
(int $errno, string $errstr, string $errfile, int $errline): bool.

This resolves the PHPStan error:
“Parameter #1 $callback of function set_error_handler expects
(callable(int, string, string, int): bool)|null, Closure(): null given.”

The handler now returns a boolean and no longer violates the callable contract.

Author: megyptm

src/Core/IntegrationValidator.php

@@ -86,7 +86,11 @@ public static function checkTimezone(): bool
      */
     public static function checkHandlers(): bool
     {
-        return is_callable(set_error_handler(fn () => null));
+        $handler = static function (int $errno, string $errstr, string $errfile, int $errline): bool {
+            return true;
+        };
+
+        return is_callable(set_error_handler($handler));
     }
 
     /**