fix(vector): address PR review round 12 — bool guard + strict doc_id validation

- _group_int_doc_ids: use type(value) is not int instead of isinstance,
  since bool is an int subclass and would otherwise stringify to
  "True"/"False" and corrupt legacy payloads on backfill.
- Replace doc_id.isdigit() guards in 5 boundary sites
  (api/visualization, auth/viz_routes, search/context note/news_item/
  deck_card branches) with a shared is_valid_nextcloud_doc_id helper
  that rejects "0", leading zeros, and Unicode digit classes
  (superscripts, Arabic-Indic, Devanagari) which pass isdigit() but
  cannot be valid MySQL AUTO_INCREMENT IDs.

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

nextcloud_mcp_server/utils/validation.py

@@ -0,0 +1,15 @@
+"""Shared validators for primitive types crossing system boundaries."""
+
+import re
+
+# Nextcloud object IDs are unsigned ints from MySQL AUTO_INCREMENT, which
+# starts at 1. Restrict to ASCII positive integers to exclude Unicode digit
+# classes (e.g. superscripts, Arabic-Indic numerals) that pass str.isdigit()
+# / str.isdecimal() but would never be valid Nextcloud IDs, and to reject "0"
+# and leading zeros.
+_NEXTCLOUD_DOC_ID_RE = re.compile(r"^[1-9][0-9]*$")
+
+
+def is_valid_nextcloud_doc_id(value: str) -> bool:
+    """True iff `value` is the str form of a positive ASCII integer (>= 1)."""
+    return bool(_NEXTCLOUD_DOC_ID_RE.fullmatch(value))