fix(example): point OPA at the in-network NATS service name

The docker-compose example pointed OPA at nats://localhost:4222, but
inside the OPA container `localhost` is the OPA container itself — not
the NATS container. OPA therefore failed at startup with:

  Failed to connect to NATS: ... nats: no servers available for connection

and exited before opening the HTTP server, which is why TestIntegration
in CI got "context deadline exceeded" while waiting for /health.

Use the docker-compose service name `nats` so OPA can reach NATS via
the docker network. A comment notes the standalone (host) value.

Author: omer9564

examples/opa-nats/config.yaml

@@ -1,6 +1,9 @@
 plugins:
   nats:
-    server_url: "nats://localhost:4222"
+    # Inside the docker-compose stack OPA reaches NATS via the docker network
+    # service name `nats`. If you run OPA outside docker (directly on the
+    # host), set this to nats://localhost:4222 instead.
+    server_url: "nats://nats:4222"
     ttl: "10m"
     refresh_interval: "30s"
     max_reconnect_attempts: 10