[php-src] PR #23500: [SimpleXML] Fix creating new attributes via attributes() dimension write

Pull Request: https://github.com/php/php-src/pull/23500 Author: iliaal Dimension writes like $x->attributes()["new"] = "v" did nothing for a not-yet-existing attribute, because sxe_prop_dim_write() replaced the element node with the first attribute node while resolving an SXE_ITER_ATTRLIST iterator, so xmlNewProp() hit a non-element node or never ran when the element had no attributes. sxe_prop_dim_write() now keeps the element node and resolves only the start of the attribute list, making dimension writes on attributes() behave like $x["new"] = "v". Property writes share this path and are fixed too; read, exists, and unset create nothing and are unaffected.