Check drain closure without assuming cancellation task order

Idle socket EOF and the draining notice do not acknowledge listener drop.
A TCP handshake may complete before the independently aborted listener is
polled; require that connection to close, and check refusal after exit.
Keep active-handler/background completion and listener liveness checks.
Reuse the existing socket deadline, with no connect retries.

Card: SAU-392

sdk/tests/wasi.rs

@@ -267,7 +267,21 @@ fn protocol() {
     assert_eq!(provider.read()["params"]["name"], "draining");
     line.clear();
     assert_eq!(idle.read_line(&mut line).unwrap(), 0);
-    assert!(std::net::TcpStream::connect(&address).is_err());
+    // drain() wakes independent accept/read cancellations. Idle EOF does not
+    // order listener destruction: TCP may still complete a handshake, but that
+    // connection must close without serving work. Do not retry the connect.
+    if let Ok(mut late) = std::net::TcpStream::connect(&address) {
+        eprintln!("TCP handshake raced listener cancellation; waiting for close");
+        // Reuse the four-second socket bound used by idle/active above.
+        late.set_read_timeout(Some(std::time::Duration::from_secs(4)))
+            .unwrap();
+        let closed = std::io::Read::read(&mut late, &mut [0]);
+        assert!(
+            matches!(closed, Ok(0))
+                || matches!(&closed, Err(e) if e.kind() == std::io::ErrorKind::ConnectionReset),
+            "late connection survived drain: {closed:?}"
+        );
+    }
     assert!(provider.child.try_wait().unwrap().is_none());
     provider.answer(&inflight, json!({}));
     line.clear();
@@ -279,6 +293,7 @@ fn protocol() {
     assert!(provider.child.try_wait().unwrap().is_none());
     provider.answer(&background, json!({}));
     provider.finishes();
+    assert!(std::net::TcpStream::connect(&address).is_err());
     assert!(
         !dir.join("stopped").exists(),
         "drain is not the EOF stop hook"