feat: add comprehensive forward handler e2e tests

Add TCP/UDP forward handler e2e tests covering basic forwarding, alias
(forward/tcp), sniffing, raw pipe, idleTimeout, multi-node protocol
filtering, sniffing bypass (403), stateful UDP, and stateless UDP.

New files:
- forward_test.go: ForwardSuite with 9 test methods
- testdata/forward/: server configs (tcp, udp, stateless, sniffing,
  bypass, multi-node, idleTimeout)
- scripts/tcp_idle_timeout.py, udp_forward_test.py: Python harnesses
  for in-container idle timeout and UDP echo verification

tests/e2e/scripts/udp_forward_test.py

@@ -0,0 +1,25 @@
+import socket
+import sys
+
+
+def main():
+    host = sys.argv[1] if len(sys.argv) > 1 else "127.0.0.1"
+    port = int(sys.argv[2]) if len(sys.argv) > 2 else 9000
+
+    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+    sock.settimeout(5)
+
+    payload = b"hello-gost-udp"
+    sock.sendto(payload, (host, port))
+
+    data, addr = sock.recvfrom(2048)
+    if data == payload:
+        print("PASS: received echo")
+        sys.exit(0)
+    else:
+        print(f"FAIL: expected {payload!r}, got {data!r}")
+        sys.exit(1)
+
+
+if __name__ == "__main__":
+    main()