sandbox: remove dead seatbelt helper and update tests (#17859)

## Why

`spawn_command_under_seatbelt()` in `codex-rs/core/src/seatbelt.rs` had
fallen out of production use and was only referenced by test-only
wrappers. That left us with sandbox tests that could stay green even if
the actual seatbelt exec path regressed, because production shell
execution now flows through `SandboxManager::transform()` and
`ExecRequest::from_sandbox_exec_request()` instead of that helper.

Removing the dead helper also exposed one downstream `codex-exec`
integration test that still imported it, which broke `just clippy`.

## What Changed

- Removed `codex-rs/core/src/seatbelt.rs` and stopped exporting
`codex_core::seatbelt`.
- Removed the redundant `codex-rs/core/tests/suite/seatbelt.rs` coverage
that only exercised the dead helper.
- Kept the `openpty` regression check, but moved it into
`codex-rs/core/tests/suite/exec.rs` so it now runs through
`process_exec_tool_call()`.
- Fixed the seatbelt denial test in `codex-rs/core/tests/suite/exec.rs`
to use `/usr/bin/touch`, so it actually exercises the sandbox instead of
a nonexistent path.
- Updated `codex-rs/exec/tests/suite/sandbox.rs` on macOS to build the
sandboxed command through `build_exec_request()` and spawn the
transformed command, instead of importing the removed helper.
- Left the lower-level seatbelt policy coverage in
`codex-rs/sandboxing/src/seatbelt_tests.rs`, where the policy generator
is still covered directly.

## Verification

- `cargo test -p codex-core suite::exec::`
- `cargo test -p codex-exec`
- `cargo clippy -p codex-exec --tests -- -D warnings`

codex-rs/core/tests/suite/exec.rs

@@ -1,8 +1,5 @@
 #![cfg(target_os = "macos")]
 
-use std::collections::HashMap;
-use std::string::ToString;
-
 use codex_core::exec::ExecCapturePolicy;
 use codex_core::exec::ExecParams;
 use codex_core::exec::process_exec_tool_call;
@@ -17,6 +14,7 @@ use codex_protocol::protocol::SandboxPolicy;
 use codex_sandboxing::SandboxType;
 use codex_sandboxing::get_platform_sandbox;
 use core_test_support::PathExt;
+use std::collections::HashMap;
 use tempfile::TempDir;
 
 fn skip_test() -> bool {
@@ -29,14 +27,18 @@ fn skip_test() -> bool {
 }
 
 #[expect(clippy::expect_used)]
-async fn run_test_cmd(tmp: TempDir, cmd: Vec<&str>) -> Result<ExecToolCallOutput> {
+async fn run_test_cmd<I, S>(tmp: TempDir, command: I) -> Result<ExecToolCallOutput>
+where
+    I: IntoIterator<Item = S>,
+    S: Into<String>,
+{
     let sandbox_type = get_platform_sandbox(/*windows_sandbox_enabled*/ false)
         .expect("should be able to get sandbox type");
     assert_eq!(sandbox_type, SandboxType::MacosSeatbelt);
     let cwd = tmp.path().abs();
 
     let params = ExecParams {
-        command: cmd.iter().map(ToString::to_string).collect(),
+        command: command.into_iter().map(Into::into).collect(),
         cwd: cwd.clone(),
         expiration: 1000.into(),
         capture_policy: ExecCapturePolicy::ShellTool,
@@ -129,6 +131,37 @@ async fn exit_command_not_found_is_ok() {
     run_test_cmd(tmp, cmd).await.unwrap();
 }
 
+#[tokio::test]
+async fn openpty_works_under_real_exec_seatbelt_path() {
+    if skip_test() {
+        return;
+    }
+
+    let python = match which::which("python3") {
+        Ok(path) => path,
+        Err(_) => {
+            eprintln!("python3 not found in PATH, skipping test.");
+            return;
+        }
+    };
+
+    let tmp = TempDir::new().expect("should be able to create temp dir");
+    let cmd = vec![
+        python.to_string_lossy().into_owned(),
+        "-c".to_string(),
+        r#"import os
+
+master, slave = os.openpty()
+os.write(slave, b"ping")
+assert os.read(master, 4) == b"ping""#
+            .to_string(),
+    ];
+
+    let output = run_test_cmd(tmp, cmd).await.unwrap();
+    assert_eq!(output.stdout.text, "");
+    assert_eq!(output.stderr.text, "");
+}
+
 /// Writing a file fails and should be considered a sandbox error
 #[tokio::test]
 async fn write_file_fails_as_sandbox_error() {
@@ -139,7 +172,7 @@ async fn write_file_fails_as_sandbox_error() {
     let tmp = TempDir::new().expect("should be able to create temp dir");
     let path = tmp.path().join("test.txt");
     let cmd = vec![
-        "/user/bin/touch",
+        "/usr/bin/touch",
         path.to_str().expect("should be able to get path"),
     ];