[codex] Keep HK snapshot generation diagnostics

[Pigbibi]

Summary

• keep generated input/artifact upload paths available even when LongBridge generation fails
• print a sanitized generation summary with row counts and failed-symbol previews
• allow failure artifacts to upload with if: always() for troubleshooting without logging credentials

Validation

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/build-hk-low-vol-snapshot-artifacts.yml"); puts "yaml ok"'
• .venv/bin/python -m pytest -q tests/test_build_hk_low_vol_snapshot_workflow.py
• .venv/bin/python -m ruff check .
• .venv/bin/python -m pytest -q tests