Skip and warn instead of stringifying floats / unexpected types in the
backfill helper. A stray doc_id=3.0 would otherwise be rewritten to
"3.0", which producers (str(int)) and the keyword index would never
match, and which int() on the verification side would reject. Also add
a doc_id=0 case to the backfill test to guard against a future
falsy-skip regression.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
