[php-src] PR #23620: Preserve array internal pointer during copy-on-write separation

Pull Request: https://github.com/php/php-src/pull/23620 Author: darkdi Fix array internal pointer reset during copy-on-write separation When an array's internal pointer is past the last element, zend_array_dup() currently clamps it to zero for packed and mixed arrays. A subsequent write that separates a copy therefore makes key() and current() observe the first element, even though the source array was already past the end. The immutable duplication path preserves nInternalPointer verbatim. Keep the same behavior in the packed and mixed mutable paths. Add a PHPT covering both representations. The test reproduces the issue on the current PHP binary before the fix. git diff --check passes; a full source build is not available in this environment. Fixes GH-23617.