Add an .isdigit() guard at the top of both chunk-context handlers so a
non-numeric doc_id fails fast with a clear 400 ("doc_id must be numeric,
got 'abc'") rather than silently bottoming out as a 404 from deep inside
get_chunk_with_context. The earlier int(doc_id) coercion was removed when
doc_id became a pure pass-through to Qdrant's keyword payload index, which
also dropped this boundary validation.
Also align test_backfill_emits_progress_log_every_20_batches' scroll stub
with real Qdrant: next_offset is now "next-1" (str) instead of 1 (int),
matching the sibling test_backfill_rewrites_int_doc_ids_to_str. Pure
stub-fidelity fix; production code already treats next_offset as opaque.
Addresses both 🟡 Important items from PR #773 review round 10.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
