fix(api): validate doc_id at chunk-context handler boundary

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>

tests/unit/vector/test_qdrant_client.py

@@ -577,8 +577,11 @@ async def test_backfill_emits_progress_log_every_20_batches(mocker, caplog):
     # set_payload calls happen — the test focuses on the progress log
     # cadence, not the rewrite path.
     str_point = SimpleNamespace(id=1, payload={"doc_id": "abc"})
-    batches: list[tuple[list[SimpleNamespace], int | None]] = [
-        ([str_point], 1) for _ in range(21)
+    # Real Qdrant returns next_offset as a UUID string (or None to terminate).
+    # Match that shape so the stub remains accurate if scroll's return type is
+    # ever tightened — and aligns with test_backfill_rewrites_int_doc_ids_to_str.
+    batches: list[tuple[list[SimpleNamespace], str | None]] = [
+        ([str_point], "next-1") for _ in range(21)
     ] + [([], None)]
     client.scroll.side_effect = batches