Address PR #272 review:
- get_md_page_content / retrieve._get_md_page_content returned every node
whose line_num fell in [min(pages), max(pages)], so a non-contiguous spec
like "5,100" over-fetched everything in between. Match the exact requested
line numbers instead, mirroring the PDF path. (Same bug as #280.)
- Restore the CHATGPT_API_KEY -> OPENAI_API_KEY backward-compat alias dropped
when pageindex/utils.py became a re-export shim; users with only
CHATGPT_API_KEY set would otherwise fail auth after upgrading. It now runs
in __init__.py right after load_dotenv.
Adds regression tests for both.
Claude-Session: https://claude.ai/code/session_01Kx5DgKbhK1N8autqXH8SmS
