[php-src] PR #23599: simplexml: filter addChild() result by the created element's namespace

Pull Request: https://github.com/php/php-src/pull/23599 Author: iliaal SimpleXMLElement::addChild() built the returned element with the prefix the caller passed in rather than the one the new node ended up in, so a child added through that returned object was filtered against the wrong namespace and could not be reached by property name. Taking the prefix from newnode->ns fixes all three cases: an explicit a:kid with a matching uri, a bare kid with a uri, and a bare kid under a prefixed parent where xmlNewChild inherits the parent's prefix. The filter keys on the prefix rather than the href deliberately. node_as_zval() installs no filter at all when the prefix is null or empty, so switching to href matching would start filtering the default-namespace case that currently has none.