Fix TOC handling, retries, and config/docs

[DankerMu]

Summary

Batch fixes for several open issues in the open-source repo (no merge requested).

Changes

• Use process_toc_no_page_numbers when TOC exists but has no page indices (addresses code no used def process_toc_no_page_numbers #30)
• Add a real retry cap to extract_toc_content to prevent infinite loops (addresses fix: prevent infinite loop in extract_toc_content #64)
• Fix fix_incorrect_toc list index shadowing bug (addresses fix: prevent list_index variable shadowing in fix_incorrect_toc #66)
• Avoid KeyError in process_none_page_numbers by using pop() (addresses fix: use pop() instead of del in process_none_page_numbers #69)
• Make ChatGPT_API_with_finish_reason return a consistent (content, finish_reason) tuple on error (addresses fix: ChatGPT_API_with_finish_reason returns inconsistent type on error #62)
• Cache OpenAI clients (sync/async) + avoid mutating chat_history (addresses Reuse OpenAI clients to improve throughput #71)
• Add missing utils.create_node_mapping() used by notebooks (addresses no create_node_mapping() in utils #41)
• Replace PyPDF2==3.0.1 with pypdf>=4.0.0 + compatibility import (addresses 🔒 Argus Security Scan Report - AI-Powered Analysis Findings #79)
• Honor --max-tokens-per-node for page grouping to reduce context overflow risk (addresses Issue: Token limit not enforced → tokens_limit_reached + unpacking crash #15)
• Sanitize non-monotonic TOC page numbers before offsetting (addresses TOC page numbers reset, spike unexpectedly, and are non-continuous in some cases #55)
• Avoid hard failures in meta_processor by logging and falling back to a minimal 1-node structure (addresses Exception: Processing failed #39)
• README: document --if-add-node-text, fix default for --if-add-doc-description, and note quoting paths with spaces (addresses run_pageindex.py doesn't return the text of the content and has wrong indices #42, If the file name has Spaces, it will simply fail #7)
• Config: allow default model via .env (CHATGPT_MODEL) and OpenAI-compatible endpoints via CHATGPT_BASE_URL

Testing

• python -m py_compile pageindex/utils.py pageindex/page_index.py pageindex/page_index_md.py run_pageindex.py
• python run_pageindex.py --md_path /tmp/pageindex_sample.md --if-add-node-summary no --if-add-doc-description no (no API calls)