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.yaml

@@ -0,0 +1,11 @@
+services:
+  - name: forward-tcp
+    addr: :8000
+    handler:
+      type: tcp
+    listener:
+      type: tcp
+    forwarder:
+      nodes:
+        - name: echo
+          addr: tcp-echo:5678

tests/e2e/testdata/forward/server_bypass_sniffing.yaml

@@ -0,0 +1,20 @@
+services:
+- name: forward-tcp
+  addr: :8000
+  handler:
+    type: tcp
+    metadata:
+      sniffing: true
+      sniffing.timeout: 2s
+  listener:
+    type: tcp
+  bypass: block-all
+  forwarder:
+    nodes:
+    - name: echo
+      addr: tcp-echo:5678
+
+bypasses:
+- name: block-all
+  matchers:
+  - 0.0.0.0/0

tests/e2e/testdata/forward/server_idle_timeout.yaml

@@ -0,0 +1,13 @@
+services:
+  - name: forward-tcp
+    addr: :8000
+    handler:
+      type: tcp
+      metadata:
+        idleTimeout: 3s
+    listener:
+      type: tcp
+    forwarder:
+      nodes:
+        - name: echo
+          addr: tcp-echo:5678

tests/e2e/testdata/forward/server_multi_node.yaml

@@ -0,0 +1,18 @@
+services:
+- name: forward-tcp
+  addr: :8000
+  handler:
+    type: tcp
+    metadata:
+      sniffing: true
+      sniffing.timeout: 2s
+  listener:
+    type: tcp
+  forwarder:
+    nodes:
+    - name: echo-http
+      addr: tcp-echo:5678
+      protocol: http
+    - name: echo-tls
+      addr: tcp-echo:1
+      protocol: tls

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

tests/e2e/testdata/forward/server_udp.yaml

@@ -0,0 +1,11 @@
+services:
+- name: udp-forward
+  addr: :9000
+  handler:
+    type: udp
+  listener:
+    type: udp
+  forwarder:
+    nodes:
+    - name: echo
+      addr: udp-echo:5679

tests/e2e/testdata/forward/server_udp_stateless.yaml

@@ -0,0 +1,15 @@
+services:
+- name: udp-forward
+  addr: :9000
+  handler:
+    type: udp
+    metadata:
+      stateless: true
+  listener:
+    type: udp
+    metadata:
+      stateless: true
+  forwarder:
+    nodes:
+    - name: echo
+      addr: udp-echo:5679