test: protect durable behavior identified in review

crates/irtt-server/tests/protocol_contracts.rs

@@ -316,3 +316,42 @@ fn negotiated_timestamp_modes_emit_only_the_selected_fields_in_order() {
         }
     }
 }
+
+#[test]
+fn duration_close_waits_for_rate_allowance_after_the_deadline() {
+    let refill = Duration::from_secs(4);
+    let maximum = Duration::from_millis(1);
+    let mut server = ServerCore::new(
+        ServerConfig::default()
+            .with_min_send_interval(refill)
+            .with_burst_allowance(1)
+            .with_idle_timeout(Duration::MAX)
+            .with_max_test_duration(maximum),
+    );
+    let opened = open(
+        &mut server,
+        &Params {
+            received_stats: ReceivedStats::Both,
+            ..Params::with_protocol_defaults()
+        },
+    );
+    let first = echo(&mut server, &opened, 0).unwrap();
+    assert_eq!(first.flags & FLAG_CLOSE, 0);
+    thread::sleep(maximum + Duration::from_secs(2));
+    assert!(
+        echo(&mut server, &opened, 63).is_none(),
+        "exhausted allowance must suppress the deadline-crossing reply"
+    );
+    assert_eq!(
+        server.session_count(),
+        1,
+        "rate refusal must not release the session"
+    );
+    thread::sleep(refill);
+    let closed = echo(&mut server, &opened, 1).unwrap();
+    assert_ne!(closed.flags & FLAG_CLOSE, 0);
+    assert_eq!(closed.sequence, 1);
+    assert_eq!(closed.recv_count, Some(2));
+    assert_eq!(closed.recv_window, Some(3));
+    assert_eq!(server.session_count(), 0);
+}