fix(ics): drop test-only imports already provided by use super::*

CI compiles the test build with RUSTFLAGS=-D warnings; the two explicit
tokio::io trait imports in the modbus test module were redundant with
`use super::*` and failed the "Main Tests" step as unused imports.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

compliance-agent/src/pipeline/ics/modbus.rs

@@ -134,7 +134,6 @@ fn parse_device_id(pdu: &[u8]) -> Option<DeviceId> {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use tokio::io::AsyncReadExt as _;
     use tokio::net::TcpListener;
 
     /// A one-shot mock Modbus/TCP server that answers a Read Holding Registers
@@ -168,7 +167,6 @@ mod tests {
                 frame.extend_from_slice(&len.to_be_bytes());
                 frame.push(hdr[6]);
                 frame.extend_from_slice(&reply_pdu);
-                use tokio::io::AsyncWriteExt as _;
                 if sock.write_all(&frame).await.is_err() {
                     break;
                 }