e2e: add UDP forward QUIC sniffing test

Add TestUDPForwardQUICSniffing: sends a real captured QUIC v1 Initial packet
through the UDP forward handler (with sniffing enabled) and verifies the
datagram is echoed back. Covers the QUIC ClientHello/ServerHello sniff path
in x/handler/redirect/udp and x/handler/forward/local.

tests/e2e/testdata/forward/server_udp_sniffing_quic.yaml

@@ -0,0 +1,15 @@
+services:
+  - name: udp-forward-quic
+    addr: :9000
+    handler:
+      type: udp
+      metadata:
+        sniffing: true
+        sniffing.timeout: 5s
+        stateless: true
+    listener:
+      type: udp
+    forwarder:
+      nodes:
+        - name: echo
+          addr: udp-echo:5679