## Problem
When uploading `.md` files with `parser=naive` and `delimiter="\n"`,
markdown headers (e.g., `## Quick Travel`) become separate chunks with
very short content (16-18 characters). This causes retrieval issues:
when the header is matched, the corresponding body text is not included
in the chunk.
## Related Issues
Closes#15487
## Checklist
- [x] Code changes are minimal and focused
- [x] Unit tests added (12/12 passed)
- [x] No breaking changes
