test: cover chunker tail flush after rollback

[haosenwang1018]

Summary

• add coverage for the _split_large_section() path where the last oversized line is rolled back and emitted during the final flush
• verify the tail line is not silently dropped after line-boundary splitting
• keep the patch scoped to chunker edge-case coverage under issue test: improve test coverage across the project #114

Testing

• uv run python -m pytest tests/test_chunker.py -q
• uv run ruff check tests/test_chunker.py
• uv run ruff format --check tests/test_chunker.py

Refs #114