Improve code quality and test robustness:
- Refactor environment variable parsing in rerank config using centralized get_env_value helper
- Add return type hints to all test methods for better type safety
- Fix patch path in test from lightrag.utils to lightrag.rerank for correct import location
- Clarify batch insert endpoint behavior regarding duplicate content rejection
- Expand .dockerignore to comprehensively exclude node_modules (200MB+), Python cache files, and venv directories
- Update dependency groups: align evaluation and test extras with pytest/pre-commit/ruff tools
