feat(sftp): add macOS and Windows platform support (#3372)

The session watchdog now selects its detection method per
platform. It previously probed kernel TCP state through a
Linux-only procfs path, so on macOS every healthy idle session
was killed within a minute, and on Windows the watchdog never
spawned at all, leaving wedged sessions with no cleanup. Linux
keeps its fast-kill watchdog unchanged. Other platforms get a
silence-only backstop that kills a session only at the
documented 30-minute idle ceiling.

The host-key loader now has a Windows arm. It loads OpenSSH
format host keys from the configured directory and logs a
one-time warning to restrict NTFS ACLs on the key directory,
the same operator-managed approach FTPS, WebDAV, KMS, and IAM
already use on Windows. Startup previously aborted with
UnsupportedPlatform because the Unix mode-bit permission check
has no Windows equivalent. tokio's io-uring feature is now
enabled only in Linux builds. io-uring is a Linux kernel
interface and enabling it unconditionally broke the Windows
build.

Co-authored-by: houseme <housemecn@gmail.com>

scripts/windows-sftp-listener-smoke.sh

@@ -0,0 +1,76 @@
+#!/usr/bin/env bash
+# Confirms rustfs.exe with --features sftp binds an SFTP listener on Windows.
+# Checks the listener bind and the warn-once host-key log line. S3 traffic is out of scope.
+
+set -euo pipefail
+
+TMP="$(mktemp -d -t rustfs-sftp-smoke-XXXXXX)"
+trap 'rm -rf "$TMP" 2>/dev/null || true' EXIT
+
+# Generate an Ed25519 host key in OpenSSH PEM format.
+ssh-keygen -t ed25519 -f "$TMP/ssh_host_ed25519_key" -N "" -q
+
+export RUSTFS_SFTP_ENABLE=true
+export RUSTFS_SFTP_ADDRESS=127.0.0.1:0
+export RUSTFS_SFTP_HOST_KEY_DIR="$TMP"
+export RUSTFS_VOLUMES="$TMP/data"
+export RUSTFS_ACCESS_KEY=smoketest
+export RUSTFS_SECRET_KEY=smoketestsecret
+# Bind S3 to an ephemeral port and skip the console so the smoke does not
+# depend on fixed ports being free on the runner.
+export RUSTFS_ADDRESS=127.0.0.1:0
+export RUSTFS_CONSOLE_ENABLE=false
+# Raise the log level so the smoke can grep the server log for the SFTP
+# listener bind line and the warn-once host-key line. rustfs defaults to
+# the error level, which would suppress both (they log at info and warn).
+export RUST_LOG=info
+mkdir -p "$RUSTFS_VOLUMES"
+
+# Launch the server in the background. The CI step that runs this script
+# has already built the rustfs.exe binary with the sftp feature. The
+# RUSTFS_BIN override lets the CI step point at a non-default location.
+BIN="${RUSTFS_BIN:-target/release/rustfs.exe}"
+if [ ! -x "$BIN" ]; then
+    echo "FAIL: $BIN is not an executable binary" >&2
+    exit 1
+fi
+"$BIN" server "$TMP/data" >"$TMP/server.log" 2>&1 &
+PID=$!
+trap 'kill "$PID" 2>/dev/null || true; rm -rf "$TMP" 2>/dev/null || true' EXIT
+
+# Discover the bound ephemeral port from the server log.
+# rustfs logs "SFTP server listening" with bind_addr 127.0.0.1:<port>
+# at startup. Grepping the log avoids the PID mismatch between the
+# MSYS-internal $! and netstat's native Windows PID under Git Bash.
+PORT=""
+for _ in $(seq 1 60); do
+    sleep 1
+    LINE="$(grep "SFTP server listening" "$TMP/server.log" 2>/dev/null | head -1 || true)"
+    if [ -n "$LINE" ]; then
+        PORT="$(echo "$LINE" | grep -oE "127\.0\.0\.1:[0-9]+" | head -1 | awk -F: '{print $NF}' || true)"
+        if [ -n "$PORT" ]; then
+            break
+        fi
+    fi
+done
+
+if [ -z "$PORT" ]; then
+    echo "FAIL: rustfs.exe did not bind any 127.0.0.1 LISTENING port within 60s" >&2
+    echo "--- server.log tail ---" >&2
+    tail -50 "$TMP/server.log" >&2 || true
+    exit 1
+fi
+
+echo "PASS: SFTP listener bound at 127.0.0.1:${PORT} (PID ${PID})"
+
+# Confirm the warn-once log line appeared in the server log.
+if ! grep -q "host key file permission enforcement is not active on Windows" \
+        "$TMP/server.log"; then
+    echo "FAIL: expected warn-once log line not found in server log" >&2
+    echo "--- server.log tail ---" >&2
+    tail -50 "$TMP/server.log" >&2 || true
+    exit 1
+fi
+
+echo "PASS: warn-once Windows host-key log line present"
+exit 0