[codex] honor sync exec stdio result shape

[andrewtdiz]

Summary

• Read explicit stdio options for execSync() and execFileSync() sync execution.
• Return null for non-piped stdout and preserve null stdout/stderr/output slots on thrown sync errors.
• Add a parity fixture for ignored/inherited sync stdio and mixed captured/null error output.

Validation

• npm exec --yes --package=node@26 -- node --experimental-strip-types test-parity/node-suite/child_process/sync/sync-stdio.ts
• npm exec --yes --package=node@26 -- bash -lc './run_parity_tests.sh --suite node-suite --module child_process --filter 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