test(integration): address round-4 review — type hints & small robustness

- Type the new helper signatures (CLAUDE.md A5): `mcp_client: Any` in
  document_is_searchable and `nc_mcp_client: Any` in _top_score.
- _top_score: guard the results list directly (`if not results`) instead of via
  total_found, so max() can't hit an empty sequence.
- _get_with_retry: replace `raise last_exc  # type: ignore` with an explicit
  `assert last_exc is not None` then raise — clearer intent, no suppressor.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/integration/_search_helpers.py

@@ -6,12 +6,13 @@ the single-user sampling tests can import it.
 
 import json
 import logging
+from typing import Any
 
 logger = logging.getLogger(__name__)
 
 
 async def document_is_searchable(
-    mcp_client, search_term: str, note_id: int | None = None
+    mcp_client: Any, search_term: str, note_id: int | None = None
 ) -> bool:
     """Return True once a freshly-created document is retrievable.