test(disco): add comprehensive XEP-0030 functional tests

Add 7 new tests for /disco command:
- disco_items_to_jid: query items to specific JID
- disco_info_empty_result: handle empty disco#info response
- disco_info_multiple_identities: multiple identity elements
- disco_info_without_name: identity without optional name attr
- disco_items_without_name: items without optional name attr
- disco_info_service_unavailable: error handling for info
- disco_items_error_handling: error handling for items (XEP-0030 §7)

The disco_items_error_handling test documents a bug where disco#items
errors are silently ignored (unlike disco#info which handles them).
This violates XEP-0030 Section 7 which requires error feedback to user.

tests/functionaltests/test_disco.h

@@ -11,3 +11,10 @@ void disco_info_not_found(void **state);
 void disco_items_shows_items(void **state);
 void disco_items_empty_result(void **state);
 void disco_requires_connection(void **state);
+void disco_items_to_jid(void **state);
+void disco_info_empty_result(void **state);
+void disco_info_multiple_identities(void **state);
+void disco_info_without_name(void **state);
+void disco_items_without_name(void **state);
+void disco_info_service_unavailable(void **state);
+void disco_items_error_handling(void **state);