fix(fs): throw opendir errors for invalid paths

[andrewtdiz]

Summary

• validate fs.opendirSync() path arguments before decoding
• surface read_dir failures as Node-shaped opendir fs errors instead of returning empty Dir objects
• route callback fs.opendir() through the same helper so regular-file paths callback with ENOTDIR
• add focused parity coverage for missing dirs, regular files, invalid path values, and valid opendir behavior

Closes #2943.

Tests

• PATH=/home/github-runner/actions-runner/externals/node24/bin:$PATH node --experimental-strip-types test-parity/node-suite/fs/readdir/opendir-invalid-paths.ts
• Pre-fix focused parity failed with opendirSync missing no-throw true: test-parity/reports/parity_report_20260530_033305.json
• PATH=/home/github-runner/actions-runner/externals/node24/bin:$PATH RUSTC_WRAPPER= ./run_parity_tests.sh --suite node-suite --module fs/readdir --filter opendir
• cargo fmt --all -- --check
• jq empty test-parity/known_failures.json test-parity/threshold.json
• git diff --check && git diff --cached --check
• ./scripts/check_file_size.sh
• RUSTC_WRAPPER= cargo check -p perry-runtime