[codex] Make unified exec tests remote aware

[pakrym-oai]

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