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/mod.rs

@@ -85,7 +85,6 @@ mod rmcp_client;
 mod rollout_list_find;
 mod safety_check_downgrade;
 mod search_tool;
-mod seatbelt;
 mod shell_command;
 mod shell_serialization;
 mod shell_snapshot;