fix(vector): address PR review round 13 — index offset fields + tighten test

- Add chunk_start_offset / chunk_end_offset to _PAYLOAD_INDEX_FIELDS so
  the legacy offset-based fallback in search/context.py works on Qdrant
  Cloud strict mode (pre-#75 clients have no chunk_index payload).
- Cover chunk_index / chunk_start_offset / chunk_end_offset in the
  payload-index summary test; refresh the stale field-list comment.
- Flag the is_valid_nextcloud_doc_id gate at both chunk-context handler
  sites with a TODO for future non-numeric doc_types.

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

nextcloud_mcp_server/api/visualization.py

@@ -505,6 +505,9 @@ async def get_chunk_context(request: Request) -> JSONResponse:
         # ints from MySQL auto_increment; doc_id stays a str downstream
         # (Qdrant payload index is keyword-typed). is_valid_nextcloud_doc_id
         # rejects "0", leading zeros, and Unicode digits that pass isdigit().
+        # TODO: when chunk-context support extends to non-numeric doc_types
+        # (calendar VEVENT UIDs, CardDAV hrefs, …), relax this gate or make
+        # it doc_type-aware. Today every indexed doc_type is numeric.
         if not is_valid_nextcloud_doc_id(doc_id):
             return JSONResponse(
                 {

nextcloud_mcp_server/auth/viz_routes.py

@@ -567,6 +567,9 @@ async def chunk_context_endpoint(request: Request) -> JSONResponse:
         # ints from MySQL auto_increment; doc_id stays a str downstream
         # (Qdrant payload index is keyword-typed). is_valid_nextcloud_doc_id
         # rejects "0", leading zeros, and Unicode digits that pass isdigit().
+        # TODO: when chunk-context support extends to non-numeric doc_types
+        # (calendar VEVENT UIDs, CardDAV hrefs, …), relax this gate or make
+        # it doc_type-aware. Today every indexed doc_type is numeric.
         if not is_valid_nextcloud_doc_id(doc_id):
             return JSONResponse(
                 {

nextcloud_mcp_server/vector/qdrant_client.py

@@ -31,13 +31,18 @@ logger = logging.getLogger(__name__)
 # ``_get_chunk_by_index_from_qdrant`` and ``get_chunk_bbox_and_page_from_qdrant``
 # (see search/context.py) — the always-indexed fast path that the offset-based
 # fallback exists to avoid; it has to actually be indexed for that promise to
-# hold on Qdrant Cloud strict mode.
+# hold on Qdrant Cloud strict mode. chunk_start_offset / chunk_end_offset are
+# the ints used by the legacy offset fallback in the same module — pre-#75
+# clients have no chunk_index payload, so the offset path still has to work
+# (or 400 silently and return None on Qdrant Cloud strict mode).
 _PAYLOAD_INDEX_FIELDS: dict[str, PayloadSchemaType] = {
     "doc_id": PayloadSchemaType.KEYWORD,
     "user_id": PayloadSchemaType.KEYWORD,
     "doc_type": PayloadSchemaType.KEYWORD,
     "is_placeholder": PayloadSchemaType.BOOL,
     "chunk_index": PayloadSchemaType.INTEGER,
+    "chunk_start_offset": PayloadSchemaType.INTEGER,
+    "chunk_end_offset": PayloadSchemaType.INTEGER,
 }
 
 # Sentinel point that records "this collection has been backfilled to str