fix: force linux/amd64 for tcp-echo in test compose (arm64 host compat)

cjimti/go-echo has no arm64 image; explicit platform lets Docker pull
and run it via Rosetta/qemu on Apple Silicon without unhealthy warnings.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/docker-compose.test.yml

@@ -39,6 +39,7 @@ services:
   # Listens on port 9000 and echoes back anything sent to it with a prefix.
   tcp-echo:
     image: cjimti/go-echo
+    platform: linux/amd64
     environment:
       TCP_PORT: "9000"
       NODE_NAME: "tcp-echo-ok"