### What problem does this PR solve?
Improve image table context.
Current strategy in attach_media_context:
- Order by position when possible: if any chunk has page/position info,
sort by (page, top, left), otherwise keep original order.
- Apply only to media chunks: images use image_context_size, tables use
table_context_size.
- Primary matching: on the same page, choose a text chunk whose vertical
span overlaps the media, then pick the one with the closest vertical
midpoint.
- Fallback matching: if no overlap on that page, choose the nearest text
chunk on the same page (page-head uses the next text; page-tail uses the
previous text).
- Context extraction: inside the chosen text chunk, find a mid-sentence
boundary near the text midpoint, then take context_size tokens split
before/after (total budget).
- No multi-chunk stitching: context comes from a single text chunk to
avoid mixing unrelated segments.
### Type of change
- [x] Refactoring
---------
Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>
### What problem does this PR solve?
Only support MinerU-API now, still need to complete frontend for
pipeline to allow the configuration of MinerU options.
### Type of change
- [x] Refactoring
### What problem does this PR solve?
Feat: Metadata in documents for improve the prompt #3690
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
