test(client): refactor probes and replies tests into submodules

Split the monolithic `probes_replies.rs` file into logical submodules and expose internal packet processing methods to improve test maintainability and coverage of boundary conditions.

crates/irtt-client/src/client/tests/probes_replies/reply_metadata.rs

@@ -0,0 +1,131 @@
+use super::*;
+
+#[test]
+#[cfg(all(target_os = "linux", feature = "ancillary"))]
+fn echo_reply_metadata_propagates_observed_dscp_with_ancillary() {
+    let params = default_params();
+    let server = start_fake_server(move |socket, tx| {
+        let (_, peer) = recv_request(&socket, &tx);
+        let reply = open_reply(FLAG_OPEN | FLAG_REPLY, TOKEN, &params, None);
+        socket.send_to(&reply, peer).unwrap();
+
+        let mut buf = [0_u8; 2048];
+        let (size, _) = socket.recv_from(&mut buf).unwrap();
+        tx.send(buf[..size].to_vec()).unwrap();
+        let seq = u32::from_le_bytes(buf[12..16].try_into().unwrap());
+        crate::socket_options::apply_dscp_to_socket(&socket, peer, 46).unwrap();
+        let reply_packet =
+            echo_reply_packet(TOKEN, seq, &params, &TimestampFields::default(), None);
+        socket.send_to(&reply_packet, peer).unwrap();
+    });
+    let config = ClientConfig {
+        socket_config: crate::SocketConfig {
+            recv_timeout: Some(Duration::from_millis(200)),
+            ..Default::default()
+        },
+        ..default_test_config(server.addr)
+    };
+    let mut client = Client::connect(config).unwrap();
+    assert_open_started(client.open().unwrap());
+
+    client.send_probe().unwrap();
+    thread::sleep(Duration::from_millis(50));
+
+    let events = client.recv_once().unwrap();
+    assert_eq!(events.len(), 1);
+    match &events[0] {
+        ClientEvent::EchoReply { packet_meta, .. } => {
+            let Some(traffic_class) = packet_meta.traffic_class else {
+                metadata_unavailable_skip(
+                    "echo_reply_metadata_propagates_observed_dscp_with_ancillary",
+                );
+                client.close().unwrap();
+                server.join();
+                return;
+            };
+            assert_eq!(traffic_class, 184);
+            assert_eq!(packet_meta.dscp, Some(46));
+            assert_eq!(packet_meta.ecn, Some(0));
+        }
+        other => panic!("expected EchoReply, got {other:?}"),
+    }
+
+    client.close().unwrap();
+    server.join();
+}
+
+#[test]
+#[cfg(all(target_os = "linux", feature = "ancillary"))]
+fn echo_reply_metadata_propagates_kernel_rx_timestamp_with_ancillary() {
+    let params = default_params();
+    let (mut client, server) = open_client_with_echo_server(&params);
+
+    client.send_probe().unwrap();
+    thread::sleep(Duration::from_millis(50));
+
+    let events = client.recv_once().unwrap();
+    assert_eq!(events.len(), 1);
+    match &events[0] {
+        ClientEvent::EchoReply { packet_meta, .. } => {
+            let Some(timestamp) = packet_meta.kernel_rx_timestamp else {
+                kernel_rx_timestamp_unavailable_skip(
+                    "echo_reply_metadata_propagates_kernel_rx_timestamp_with_ancillary",
+                );
+                client.close().unwrap();
+                server.join();
+                return;
+            };
+            assert!(timestamp.duration_since(SystemTime::UNIX_EPOCH).is_ok());
+        }
+        other => panic!("expected EchoReply, got {other:?}"),
+    }
+
+    client.close().unwrap();
+    server.join();
+}
+
+#[test]
+fn process_received_packet_uses_supplied_receive_metadata() {
+    let params = default_params();
+    let server = silent_open_server(params.clone());
+    let config = ClientConfig {
+        socket_config: crate::SocketConfig {
+            recv_timeout: Some(Duration::from_millis(50)),
+            ..Default::default()
+        },
+        ..default_test_config(server.addr)
+    };
+    let mut client = Client::connect(config).unwrap();
+    assert_open_started(client.open().unwrap());
+
+    client.send_probe_at(ClientTimestamp::now()).unwrap();
+    let recv_ts = ClientTimestamp::now();
+    let reply = echo_reply_packet(TOKEN, 0, &params, &TimestampFields::default(), None);
+    let events = client
+        .process_received_packet(
+            &reply,
+            recv_ts,
+            ReceiveMeta {
+                traffic_class: Some(186),
+                kernel_rx_timestamp: Some(SystemTime::UNIX_EPOCH + Duration::new(12, 34)),
+            },
+        )
+        .unwrap();
+
+    assert_eq!(events.len(), 1);
+    match &events[0] {
+        ClientEvent::EchoReply { packet_meta, .. } => {
+            assert_eq!(packet_meta.traffic_class, Some(186));
+            assert_eq!(packet_meta.dscp, Some(46));
+            assert_eq!(packet_meta.ecn, Some(2));
+            assert_eq!(
+                packet_meta.kernel_rx_timestamp,
+                Some(SystemTime::UNIX_EPOCH + Duration::new(12, 34))
+            );
+        }
+        other => panic!("expected EchoReply, got {other:?}"),
+    }
+
+    client.close().unwrap();
+    server.join();
+}