feat: infer hierarchical heading levels (H1-H6) for PDFs (#4204)

[statxc]

Summary

• Add two-strategy heading level inference for PDF Title elements via category_depth metadata
  • Outline extraction (primary): walks PDF bookmark tree and matches entries to Title elements by page number + text similarity
  • Font-size analysis (fallback): clusters distinct font sizes from pdfminer LTChar data, ranks largest-first to assign depth 0-5
• Integrates as a post-processing step in partition_pdf_or_image(), works with all strategies (fast, hi_res, ocr_only)
• Correctly skipped for image partitioning

Test plan

• Existing test_document_to_element_list_sets_category_depth_titles passes unchanged
• 154 existing PDF tests pass (1 pre-existing OCR language failure unrelated)

Closes #4204

[statxc]

Hi, @PastelStorm @cragwolfe
Could you review my PR please.
Please let me know if anything else is needed to update.
Thanks.

[codebymikey]

There are currenty some merge conflicts that need resolving.

[statxc]

@PastelStorm @codebymikey
Could you review this PR please? I'd appreciate any feedback from you. Please review my PR when you have time. Thanks.

[codebymikey]

Hi @statxc,

I'm not particularly sure I'll be able to review this effectively as I'm not a maintainer of the project or familiar enough with it to comment. I'm merely a potential end-user who pointed out the missing functionality.

Did you implement this yourself or with help from AI? One of the issues the previous PR trying to address this had was how it was hard to review because the original PR user wasn't fully aware of what the code actually did, and just made PRs based off the output of the tool.

Also, based off her Github activity, I believe @PastelStorm might be away for a while.

If you feel your PR addresses the issue to a sufficiently high standard, then feel free to ping some of the more recently active maintainers/committers for review.

[statxc]

@badGarnet @cragwolfe Hi, Sorry for ping many people. But this PR has been open for quite a while. I'd appreciate you give me any feedback.