Reject class_alias() on internal classes (#9211)

[PurHur]

Summary

• class_alias('stdClass', …) and other internal-class targets now throw ValueError with Zend-shaped message instead of registering a working alias
• VM Context::registerClassAlias checks ClassEntry::isInternal; JIT Object_::registerClassAlias rejects externalOnlyClassIds the same way
• php-src reference: Zend/zend_builtin_functions.c / ext/standard/basic_functions.c

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro/maintainer_gap_class_alias_internal.php
php bin/vm.php test/repro/maintainer_gap_class_alias_internal.php
php vendor/bin/phpunit --filter class_alias_internal
'

Output (both Zend + VM):

ValueError: class_alias(): Argument #1 ($class) must be a user-defined class name, internal class name given
false

PHPUnit: OK (2 tests, 2 assertions) (VM + JIT compliance).

Closes #9211

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