When multiple image columns are found, probes them (HEAD for URLs,
os.path.exists for paths) and picks the first that works.
Skips probing when top candidate is PIL/dict (score >= 75).
find_image_column now scores candidates by resolvability (PIL > dict > URL > path)
and has a Pass 2 value-based fallback for columns not matching image keywords.
Fixes phiyodr/coco2017 picking file_name (unresolvable) over coco_url (resolvable).
- Detect and convert ShareGPT/ChatML conversations with <image> placeholders
- Add file_name/filename as image column keywords
- Detect image paths and URLs by value (string ending in .jpg/.png/etc)
