test(core): add seatbelt sem lock tests (#1823)

## Summary
- add a unit test to ensure the macOS seatbelt policy allows POSIX
semaphores
- add a macOS-only test that runs a Python multiprocessing Lock under
Seatbelt

## Testing
- `cargo test -p codex_core seatbelt_base_policy_allows_ipc_posix_sem
--no-fail-fast` (failed: failed to download from
`https://static.crates.io/crates/tokio-stream/0.1.17/download`)
- `cargo test -p codex_core seatbelt_base_policy_allows_ipc_posix_sem
--no-fail-fast --offline` (failed: attempting to make an HTTP request,
but --offline was specified)
- `cargo test --all-features --no-fail-fast --offline` (failed:
attempting to make an HTTP request, but --offline was specified)
- `just fmt` (failed: command not found: just)
- `just fix` (failed: command not found: just)

Ran tests locally to confirm it passes on master and failed before my
previous change

------
https://chatgpt.com/codex/tasks/task_i_6890f221e0a4833381cfb53e11499bcc

codex-rs/core/src/seatbelt.rs

@@ -301,6 +301,59 @@ mod tests {
         assert_eq!(expected_args, args);
     }
 
+    #[test]
+    fn seatbelt_base_policy_allows_ipc_posix_sem() {
+        assert!(
+            MACOS_SEATBELT_BASE_POLICY.contains("(allow ipc-posix-sem)"),
+            "base policy should allow ipc-posix-sem"
+        );
+    }
+
+    #[cfg(target_os = "macos")]
+    #[tokio::test]
+    async fn python_multiprocessing_lock_works_under_seatbelt() {
+        use super::spawn_command_under_seatbelt;
+        use crate::spawn::StdioPolicy;
+        use std::collections::HashMap;
+
+        let policy = SandboxPolicy::WorkspaceWrite {
+            writable_roots: vec![],
+            network_access: false,
+            include_default_writable_roots: true,
+        };
+
+        let python_code = r#"import multiprocessing
+from multiprocessing import Lock, Process
+
+def f(lock):
+    with lock:
+        print("Lock acquired in child process")
+
+if __name__ == '__main__':
+    lock = Lock()
+    p = Process(target=f, args=(lock,))
+    p.start()
+    p.join()
+"#;
+
+        let mut child = spawn_command_under_seatbelt(
+            vec![
+                "python3".to_string(),
+                "-c".to_string(),
+                python_code.to_string(),
+            ],
+            &policy,
+            std::env::current_dir().expect("should be able to get current dir"),
+            StdioPolicy::RedirectForShellTool,
+            HashMap::new(),
+        )
+        .await
+        .expect("should be able to spawn python under seatbelt");
+
+        let status = child.wait().await.expect("should wait for child process");
+        assert!(status.success(), "python exited with {status:?}");
+    }
+
     struct PopulatedTmp {
         root_with_git: PathBuf,
         root_without_git: PathBuf,