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/testdata/forward/server_sniffing.yaml

@@ -0,0 +1,14 @@
+services:
+  - name: forward-tcp
+    addr: :8000
+    handler:
+      type: tcp
+      metadata:
+        sniffing: true
+        sniffing.timeout: 2s
+    listener:
+      type: tcp
+    forwarder:
+      nodes:
+        - name: echo
+          addr: tcp-echo:5678