fix(quic-interop): override picoquic entrypoint so smoke runs picoquicdemo

The privateoctopus/picoquic:latest image now wraps its CMD in the
runner's /run_endpoint.sh, which expects ROLE/TESTCASE/CERTS env vars
and exits 127 with "Unsupported test case:" if they're absent. So the
old `... privateoctopus/picoquic:latest picoquicdemo -p 4433` form
silently doesn't start picoquicdemo at all — picoquic exits, our
client times out trying to talk to a dead container.

Override the entrypoint so picoquicdemo runs directly with our args.

https://claude.ai/code/session_01HcvfQq1ttPV9PkRoJb4nyT

quic/interop/Makefile

@@ -38,7 +38,8 @@ smoke: docker smoke-down
 	docker network create $(SMOKE_NET) >/dev/null 2>&1 || true
 	docker run -d --name $(SMOKE_PICOQUIC) \
 	    --network $(SMOKE_NET) \
-	    privateoctopus/picoquic:latest picoquicdemo -p 4433 >/dev/null
+	    --entrypoint picoquicdemo \
+	    privateoctopus/picoquic:latest -p 4433 >/dev/null
 	@PICOQUIC_IP=$$(docker inspect -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' $(SMOKE_PICOQUIC)); \
 	    echo "picoquic at $$PICOQUIC_IP:4433 (in $(SMOKE_NET))"; \
 	    docker run --rm --name $(SMOKE_CLIENT) \