fix(optimizer): handle internal class literals in get_parent_class

[UEhQZXI]

Problem

get_parent_class() crashes during conversion when a string literal names an internal PHP class:

get_parent_class('RuntimeException');

The optimizer assumes every literal class name exists in the compilation symbol table and uses the non-null lookup, so internal classes produce an invalid lookup instead of falling back to runtime resolution.

Fix

Use the nullable class lookup for compile-time folding. Known project classes continue to fold at compile time, while internal class names use php::fn::get_parent_class().

Tests

• Added focused code-generation coverage for internal class literals.
• Extended get_parent_class() PHPT coverage for internal classes with and without parents.
• Verified the PHPUnit, PHPT, and platform build workflows on PHP 8.4 and PHP 8.5.