fix(vector): address PR review round 14 — accurate offset-skip comment + news_item doc_id guard

Round-14 review surfaced one blocking and one important issue.

context.py: the comment justifying `skip_offset_lookup` claimed
chunk_start/end_offset weren't in _PAYLOAD_INDEX_FIELDS — round 13
indexed both as INTEGER, so the comment now actively misleads. Replace
with the real reason: an indexed chunk_index miss is canonical (both
paths hit the same Qdrant collection), and skipping the offset filter
avoids a redundant round-trip.

verification.py: hoist an is_valid_nextcloud_doc_id guard before the
`int(d)` cast in _verify_news_items, mirroring the boundary-validation
pattern already in _fetch_document_text. Coerce via `str(d)` because
SearchResult.id is `int | str` (D1 forward-compat widening). Malformed
ids now surface as a logger.warning rather than a generic debug line;
fail-open semantics are preserved.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/search/context.py

@@ -374,14 +374,13 @@ async def get_chunk_with_context(
         chunk_text = await _get_chunk_by_index_from_qdrant(
             user_id, doc_id, doc_type, chunk_index
         )
-    # When chunk_index is available, treat the indexed lookup as canonical
-    # for every doc_type. A miss means the chunk is genuinely absent, not
-    # "fall back to the unindexed slow path". chunk_start/end_offset aren't
-    # in _PAYLOAD_INDEX_FIELDS, so the offset filter 400s in Qdrant Cloud
-    # strict mode and surfaces a misleading logger.error. Legacy data
-    # without chunk_index (pre-cbcoutinho/astrolabe#75) still hits the
-    # offset path and degrades to a None chunk with a WARNING; that's the
-    # same behavior get_chunk_bbox_and_page_from_qdrant already documents.
+    # When chunk_index is supplied, the indexed lookup is canonical: both the
+    # index path and the offset path query the same Qdrant collection, so an
+    # indexed miss means the chunk is genuinely absent. Skipping the offset
+    # filter avoids a redundant Qdrant round-trip. Legacy data without
+    # chunk_index (pre-cbcoutinho/astrolabe#75) still hits the offset path
+    # and degrades to a None chunk with a WARNING; that's the same behavior
+    # get_chunk_bbox_and_page_from_qdrant already documents.
     skip_offset_lookup = chunk_index is not None
     if chunk_text is None and not skip_offset_lookup:
         chunk_text = await _get_chunk_from_qdrant(