[codex] honor spawnSync stdio result shape

[andrewtdiz]

Summary

• Teach spawnSync() to read the existing child_process stdio option subset for sync execution.
• Return null for stdout, stderr, and output slots when stdout/stderr are ignore or inherit instead of boxing empty captures.
• Add a Node parity fixture for default pipe, string stdio, and mixed array stdio result shapes.

Validation

• npm exec --yes --package=node@26 -- bash -lc './run_parity_tests.sh --suite node-suite --module child_process --filter spawn-sync-stdio'
• npm exec --yes --package=node@26 -- bash -lc './run_parity_tests.sh --suite node-suite --module child_process' (18 pass, 0 fail, 0 compile fail)
• cargo fmt --all -- --check
• git diff --check
• ./scripts/check_file_size.sh