fix: fallback to Landlock-only when user namespaces unavailable and set PR_SET_NO_NEW_PRIVS early (#9250)

fixes https://github.com/openai/codex/issues/9236

### Motivation
- Prevent sandbox setup from failing when unprivileged user namespaces
are denied so Landlock-only protections can still be applied.
- Ensure `PR_SET_NO_NEW_PRIVS` is set before installing seccomp and
Landlock restrictions to avoid kernel `EPERM`/`LandlockRestrict`
ordering issues.

### Description
- Add `is_permission_denied` helper that detects `EPERM` /
`PermissionDenied` from `CodexErr` to drive fallback logic.
- In `apply_read_only_mounts` skip read-only bind-mount setup and return
`Ok(())` when `unshare_user_and_mount_namespaces()` fails with
permission-denied so Landlock rules can still be installed.
- Add `set_no_new_privs()` and call it from
`apply_sandbox_policy_to_current_thread` before installing seccomp
filters and Landlock rules when disk or network access is restricted.

codex-rs/Cargo.lock

@@ -1524,6 +1524,7 @@ dependencies = [
  "codex-utils-absolute-path",
  "landlock",
  "libc",
+ "pretty_assertions",
  "seccompiler",
  "tempfile",
  "tokio",