fix: improve test_routing_mesh and fix duplicate init_connections in test_etcp_two_instances

- Refactored test_routing_mesh to use proper ETCP + normalizer flow
- Removed duplicate init_connections() calls that caused 'Address already in use'
- Improved error reporting in etcp_link_new()
- Test now respects single mainloop architecture

src/etcp_connections.c

@@ -523,7 +523,7 @@ struct ETCP_LINK* etcp_link_new(struct ETCP_CONN* etcp, struct ETCP_SOCKET* conn
 
     struct ETCP_LINK* link = u_calloc(1, sizeof(struct ETCP_LINK));
     if (!link) {
-        DEBUG_ERROR(DEBUG_CATEGORY_CONNECTION, "etcp_link_new: calloc failed");
+        DEBUG_ERROR(DEBUG_CATEGORY_CONNECTION, "etcp_link_new: calloc failed - out of memory or pool exhausted");
         return NULL;
     }
     DEBUG_INFO(DEBUG_CATEGORY_CONNECTION, "NEW link: etcp=[%s] link=%p etcp=%p", etcp->log_name, link, etcp);