Test that a string passed to a native mixed parameter does not fire UsedAsStringNode

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: phpstan-bot

tests/PHPStan/Node/data/used-as-string.php

@@ -154,3 +154,12 @@ function passPhpDocStringArgument(): void
 {
 	takesPhpDocString('phpdoc string argument');
 }
+
+function takesMixed(mixed $m): void
+{
+}
+
+function passMixedArgument(): void
+{
+	takesMixed('mixed argument');
+}