[codex] Preserve zsh PATH in shell snapshots

[sayan-oai]

Summary

Fix zsh shell snapshot export filtering so tied parameters such as PATH/path are not dropped. zsh emits PATH from export -p as export -T PATH path=(...), and the previous parser treated -T PATH path as the variable name and filtered it out as invalid.

Add a macOS regression test that captures a zsh snapshot from a temporary .zshrc, sources the snapshot in a fresh zsh process with a deliberately wrong PATH, and verifies the captured login-shell PATH is restored.

Impact

This keeps shell snapshots from losing the login-shell PATH on zsh. That matters for extension-host or other thin-parent-environment sessions where inheriting the parent PATH does not mask a missing snapshot PATH export.

Validation

• just fmt
• cargo test -p codex-core zsh_snapshot_preserves_tied_path_export
• cargo test -p codex-core shell_snapshot::tests
• cargo test -p codex-core was also run locally, but failed in unrelated integration/approval tests after the shell snapshot unit tests passed. Failures included approval/otel cases plus shell snapshot integration tests timing out while waiting for zsh snapshot creation in this local environment.