Honor explicit Unix socket grants in the Linux managed sandbox (#45534)

## Why

Linux proxy-routed sandboxing denied standalone Unix sockets even when the effective network policy enabled `dangerously_allow_all_unix_sockets`.

## What changed

- Carry Unix socket permissions in `ManagedNetworkSandboxContext` and pass the prepared context through Linux sandbox launches with `--managed-network`.
- Allow `AF_UNIX` socket creation in proxy-routed mode when `dangerously_allow_all_unix_sockets` is enabled, while preserving network namespace isolation and restrictions on other socket families.
- Keep standalone Unix sockets denied by default and for path-only grants. Default missing fields in older serialized contexts to restrictive values.

## Testing

Add coverage for policy preparation and transport, legacy deserialization, and malformed policy rejection. Add a Linux integration test covering default denial, path-only denial, and explicit allow-all access, while checking that direct TCP access and `AF_NETLINK`/`AF_VSOCK` sockets remain blocked.

GitOrigin-RevId: 2695b945ad3e59fcb3faf7662d852a26650af16c

codex-rs/exec-server/src/process_sandbox_tests.rs

@@ -240,6 +240,7 @@ async fn sandbox_request_allows_prepared_managed_proxy_port() {
         managed_network: Some(ManagedNetworkSandboxContext {
             loopback_ports: vec![43123],
             allow_local_binding: false,
+            ..Default::default()
         }),
         network_proxy: None,
     };