[codex] Make unified exec tests remote aware (#16977)

## Summary
- Convert unified exec integration tests that can run against the remote
executor to use the remote-aware test harness.
- Create workspace directories through the executor filesystem for
remote runs.
- Install `python3` and `zsh` in the remote test container so restored
Python/zsh-based test commands work in fresh Ubuntu containers.

## Validation
- `just fmt`
- `cargo test -p codex-core --test all unified_exec_defaults_to_pipe`
- `cargo test -p codex-core --test all unified_exec_can_enable_tty`
- `cargo test -p codex-core --test all unified_exec`
- Remote on `codex-remote`: `source scripts/test-remote-env.sh && cd
codex-rs && cargo test -p codex-core --test all unified_exec`
- `just fix -p codex-core`

scripts/test-remote-env.sh

@@ -49,6 +49,10 @@ setup_remote_env() {
 
   docker rm -f "${container_name}" >/dev/null 2>&1 || true
   docker run -d --name "${container_name}" ubuntu:24.04 sleep infinity >/dev/null
+  if ! docker exec "${container_name}" sh -lc "apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y python3 zsh"; then
+    docker rm -f "${container_name}" >/dev/null 2>&1 || true
+    return 1
+  fi
 
   export CODEX_TEST_REMOTE_ENV="${container_name}"
 }