fix(vector): guard _group_int_doc_ids against non-int doc_id values

Skip and warn instead of stringifying floats / unexpected types in the
backfill helper. A stray doc_id=3.0 would otherwise be rewritten to
"3.0", which producers (str(int)) and the keyword index would never
match, and which int() on the verification side would reject. Also add
a doc_id=0 case to the backfill test to guard against a future
falsy-skip regression.

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

nextcloud_mcp_server/vector/qdrant_client.py

@@ -161,6 +161,18 @@ def _group_int_doc_ids(points: list[Any]) -> tuple[dict[str, list[Any]], int]:
         value = payload.get("doc_id")
         if value is None or isinstance(value, str):
             continue
+        if not isinstance(value, int):
+            # Producers only ever write int or str; anything else is a
+            # producer bug. Stringifying e.g. a float would write "3.0",
+            # which producers (str(int)) and the keyword index would
+            # never match, and which int() on the verification side
+            # would later reject. Skip and log loudly instead.
+            logger.warning(
+                "Unexpected doc_id type %s on point %s; skipping rewrite",
+                type(value).__name__,
+                point.id,
+            )
+            continue
         by_value.setdefault(str(value), []).append(point.id)
     return by_value, scanned