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.rs

@@ -664,6 +664,26 @@ impl Client {
     fn send_probe_at(&mut self, ts: ClientTimestamp) -> Result<Vec<ClientEvent>, ClientError> {
         self.send_probe_inner(Some(ts))
     }
+
+    fn process_received_packet(
+        &mut self,
+        packet: &[u8],
+        received_at: ClientTimestamp,
+        meta: ReceiveMeta,
+    ) -> Result<Vec<ClientEvent>, ClientError> {
+        let Some(reply) = self.decode_received_packet(packet) else {
+            return Ok(vec![ClientEvent::Warning {
+                kind: WarningKind::MalformedOrUnrelatedPacket,
+                message: "dropped malformed or unrelated packet".to_owned(),
+                at: received_at,
+            }]);
+        };
+        self.process_echo_reply(reply, packet.len(), received_at, meta)
+    }
+
+    fn recv_buffer_size(&self) -> usize {
+        self.recv_buffer.len()
+    }
 }
 
 fn update_highest_received(highest_received_seq: &mut Option<u32>, wire_seq: u32) {