Fix sandbox extraction follow-ups

- trim leftover codex-core sandbox shims and retarget the remaining type plumbing
- keep sandbox-related dependencies scoped to the crates and targets that actually use them

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/exec_tests.rs

@@ -1,4 +1,5 @@
 use super::*;
+use codex_sandbox::IO_DRAIN_TIMEOUT_MS;
 use codex_protocol::config_types::WindowsSandboxLevel;
 use pretty_assertions::assert_eq;
 use std::collections::HashMap;