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/core/src/unified_exec/process_manager_tests.rs

@@ -168,6 +168,7 @@ fn exec_server_params_use_path_uri_and_env_policy_overlay_contract() {
     let managed_network = ManagedNetworkSandboxContext {
         loopback_ports: vec![43123],
         allow_local_binding: false,
+        ..Default::default()
     };
     let mut request = ExecRequest {
         command: vec!["bash".to_string(), "-lc".to_string(), "true".to_string()],