xml2: Fix crash with namespace nodes in xpath_nodeset()

xml2: Fix crash with namespace nodes in xpath_nodeset()

pgxmlNodeSetToText() passed nodeTab[i]->doc to xmlNodeDump() without
checking the node type, which could cause a crash as a
XML_NAMESPACE_DECL maps to a xmlNs struct.  The passed-in code would
then be dereferenced in xmlNodeDump().

This commit switches the code to render XML_NAMESPACE_DECL nodes with
xmlXPathCastNodeToString(), like xpath_table().  Some tests are added,
written by me.

Author: Andrey Chernyy <andrey.cherny@tantorlabs.com>
Co-authored-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/20260611031436.5afde3cb@andrnote
Backpatch-through: 14