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(
                 {