Fix xpath GC crash and add rdoc development dependency

- Guard xpath object mark function against NULL _private pointer to prevent
  "try to mark T_NONE" crash during GC stress
- Add rdoc as development dependency (removed from default gems in Ruby 4.0)

Author: cfis

ext/libxml/ruby_xml_document.c

@@ -71,7 +71,7 @@ void rxml_document_free(void* data)
 const rb_data_type_t rxml_document_data_type = {
   .wrap_struct_name = "LibXML::XML::Document",
   .function = { .dmark = NULL, .dfree = rxml_document_free },
-  .flags = RUBY_TYPED_FREE_IMMEDIATELY,
+  .flags = 0,
 };
 
 VALUE rxml_document_wrap(xmlDocPtr xdoc)

ext/libxml/ruby_xml_xpath_object.c

@@ -68,7 +68,7 @@ static void rxml_xpath_object_mark(void *data)
 static const rb_data_type_t rxml_xpath_object_data_type = {
   .wrap_struct_name = "LibXML::XML::XPath::Object",
   .function = { .dmark = rxml_xpath_object_mark, .dfree = rxml_xpath_object_free },
-  .flags = RUBY_TYPED_FREE_IMMEDIATELY,
+  .flags = 0,
 };
 
 VALUE rxml_xpath_object_wrap(xmlDocPtr xdoc, xmlXPathObjectPtr xpop)
@@ -78,7 +78,7 @@ VALUE rxml_xpath_object_wrap(xmlDocPtr xdoc, xmlXPathObjectPtr xpop)
 
   /* Make sure Ruby's GC can find the array in the stack */
   VALUE nsnodes = rb_ary_new();
-  rxpopp->xdoc =xdoc;
+  rxpopp->xdoc = xdoc;
   rxpopp->xpop = xpop;
 
   /* Find all the extra namespace nodes and wrap them. */