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/mod.rs

@@ -0,0 +1,29 @@
+use super::*;
+use support::echo_packet_len;
+
+#[cfg(not(all(target_os = "linux", feature = "ancillary")))]
+fn assert_packet_meta_unavailable(packet_meta: &crate::event::PacketMeta) {
+    assert_eq!(packet_meta.traffic_class, None);
+    assert_eq!(packet_meta.dscp, None);
+    assert_eq!(packet_meta.ecn, None);
+    assert_eq!(packet_meta.kernel_rx_timestamp, None);
+}
+
+#[cfg(all(target_os = "linux", feature = "ancillary"))]
+fn metadata_unavailable_skip(test_name: &str) {
+    eprintln!("{test_name}: skipping metadata assertion because kernel did not provide traffic class metadata");
+}
+
+#[cfg(all(target_os = "linux", feature = "ancillary"))]
+fn kernel_rx_timestamp_unavailable_skip(test_name: &str) {
+    eprintln!("{test_name}: skipping kernel timestamp assertion because kernel did not provide SCM_TIMESTAMPNS");
+}
+
+mod burst_recv;
+mod packet_length;
+mod probe_schedule;
+mod reply_classification;
+mod reply_decode;
+mod reply_metadata;
+mod timeouts;
+mod timing_math;