[codex] Enable Node env proxy for managed network proxy (#23905)

## Summary
- set `NODE_USE_ENV_PROXY=1` when Codex applies managed network proxy
environment overrides
- keep the Node opt-in in the proxy environment key set used by
shell/runtime env handling
- cover the new env var in the focused network proxy env test

## Why
Codex already sets HTTP proxy environment variables for child processes
when the managed network proxy is active. Node's built-in network
behavior needs the `NODE_USE_ENV_PROXY` opt-in to honor those env vars,
so Node-based skill scripts can otherwise skip the managed proxy path
and fail under restricted network access.

## Validation
- `just fmt` in `codex-rs`
- `cargo test -p codex-network-proxy` in `codex-rs`

codex-rs/network-proxy/src/proxy.rs

@@ -366,12 +366,14 @@ pub const ALL_PROXY_ENV_KEYS: &[&str] = &["ALL_PROXY", "all_proxy"];
 pub const PROXY_ACTIVE_ENV_KEY: &str = "CODEX_NETWORK_PROXY_ACTIVE";
 pub const ALLOW_LOCAL_BINDING_ENV_KEY: &str = "CODEX_NETWORK_ALLOW_LOCAL_BINDING";
 const ELECTRON_GET_USE_PROXY_ENV_KEY: &str = "ELECTRON_GET_USE_PROXY";
+const NODE_USE_ENV_PROXY_ENV_KEY: &str = "NODE_USE_ENV_PROXY";
 #[cfg(any(target_os = "macos", test))]
 const GIT_SSH_COMMAND_ENV_KEY: &str = "GIT_SSH_COMMAND";
 pub const PROXY_ENV_KEYS: &[&str] = &[
     PROXY_ACTIVE_ENV_KEY,
     ALLOW_LOCAL_BINDING_ENV_KEY,
     ELECTRON_GET_USE_PROXY_ENV_KEY,
+    NODE_USE_ENV_PROXY_ENV_KEY,
     "HTTP_PROXY",
     "HTTPS_PROXY",
     "http_proxy",
@@ -525,6 +527,8 @@ fn apply_proxy_env_overrides(
         ELECTRON_GET_USE_PROXY_ENV_KEY.to_string(),
         "true".to_string(),
     );
+    // Node.js built-in HTTP clients only honor proxy environment variables when this is enabled.
+    env.insert(NODE_USE_ENV_PROXY_ENV_KEY.to_string(), "1".to_string());
 
     // Keep HTTP_PROXY/HTTPS_PROXY as HTTP endpoints. A lot of clients break if
     // those vars contain SOCKS URLs. We only switch ALL_PROXY here.
@@ -1016,6 +1020,7 @@ mod tests {
             env.get(ELECTRON_GET_USE_PROXY_ENV_KEY),
             Some(&"true".to_string())
         );
+        assert_eq!(env.get(NODE_USE_ENV_PROXY_ENV_KEY), Some(&"1".to_string()));
         #[cfg(target_os = "macos")]
         assert_eq!(
             env.get(GIT_SSH_COMMAND_ENV_KEY),