test: retain durable negotiation and lifecycle review coverage

crates/irtt-server/tests/session_behavior.rs

@@ -31,6 +31,9 @@ fn authenticated_sessions_are_bounded_and_owned_by_the_opening_endpoint() {
         Some(key),
     )
     .unwrap();
+    let mut unkeyed = ServerCore::new(ServerConfig::default());
+    assert!(unkeyed.handle_datagram(peer, &open).unwrap().is_none());
+    assert_eq!(unkeyed.session_count(), 0);
     let unauthenticated = encode_request(
         RequestToEncode::Open {
             params: &params,