Address claude-review round 1 on PR #868:
- use_page_aware now gates on `bool(page_boundaries)` instead of
`is not None`, so a PDF that yields an empty boundary list takes the
char-based path explicitly (assign_page_numbers no-ops on []) rather than
the page-aware chunker's no-boundaries fallback. Same result, clearer intent.
- add test_oversized_page_with_leading_whitespace_offsets, exercising the
start+start_index offset path for an oversized page whose sub-chunks have
leading whitespace.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>