…s in node entrypoint


This PR hardens the node's execution entrypoint by implementing strict input validation for the L2 engine secret and reducing the default unauthenticated RPC attack surface, addressing the vulnerabilities identified in the workspace security audit.

**Engine Authentication Validation:**
* Added strict fail-closed validation for `BASE_NODE_L2_ENGINE_AUTH_RAW` in `node/execution-entrypoint`. The node startup will now safely exit with an error if the provided value is absent or not exactly 64 hexadecimal characters, preventing the engine from running with malformed authentication material.

**RPC Attack Surface & Cross-Origin Protection:**
* Restricted the default privileged HTTP and WebSocket APIs. The entrypoint now securely defaults to `web3,eth,net` instead of exposing `debug`, `txpool`, and `miner` namespaces automatically.
* Replaced wildcard (`*`) HTTP CORS and WebSocket origins with a restricted default of `http://localhost`. Operators can still explicitly configure these via the `RETH_HTTP_CORS_DOMAINS` and `RETH_WS_ORIGINS` environment variables.