CVE-2024-51755: Twig has unguarded calls to `__isset()` and to array-accesses when the sandbox is enabled

The vulnerability stemmed from two key issues: 1) In CoreExtension::getAttribute, security checks for property access were performed after isset()/array access checks, potentially triggering __isset() methods before policy validation. 2) In GetAttrExpression::compile, the generated code for array accesses didn't properly restrict non-native ArrayAccess implementations. The commit diff shows security checks were moved before isset() checks and array-like class validation was added, confirming these were the vulnerable paths. The test cases added (MagicObject/ArrayLikeObject) specifically target these scenarios.