Language: deny Generator serialize/unserialize like Zend (#23044)

[PurHur]

Summary

• Reject serialize() / unserialize() of Generator with Zend-identical Exception messages (Zend/zend_generators.c serialize/unserialize deny).
• Mirror the existing Closure/Fiber deny paths in VmSerialize (encode + export + unserialize header).
• Compliance .phpt + repro; Closure deny asserted first.

Closes #23044

php-src reference

• Zend/zend_generators.c — zend_class_serialize_deny / zend_class_unserialize_deny

PHP implementation

• ext/standard/VmSerialize.php (PHP-in-PHP; no new runtime/*.c)

Test plan

• php bin/vm.php test/repro/issue_23044_generator_serialize.php
• ./script/phpunit.sh --filter generator_serialize_disallowed → OK (2 tests)
• Host PHPT expect match for generator_serialize_disallowed.phpt

Verification

$ php bin/vm.php test/repro/issue_23044_generator_serialize.php
closure:Exception:Serialization of 'Closure' is not allowed
Exception:Serialization of 'Generator' is not allowed
Exception:Unserialization of 'Generator' is not allowed

$ ./script/phpunit.sh --filter generator_serialize_disallowed
OK (2 tests, 2 assertions)

Note: full GeneratorVMTest still has a pre-existing red on generator_finally_early_close.phpt (FIN_UNSET missing) on this tree — unrelated to serialize deny.

Trust snapshot: inventory OK; north-star5-fast red on gen-0 stale (#22642) — unrelated pillar 1.