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/test_astrolabe_session_jwt_search.py

@@ -57,7 +57,8 @@ async def _get_with_retry(
                 "GET %s failed (attempt %s/%s): %s", url, attempt, max_attempts, e
             )
             await anyio.sleep(2)
-    raise last_exc  # type: ignore[misc]
+    assert last_exc is not None  # loop ran at least once, so this is set
+    raise last_exc
 
 
 async def _astrolabe_configured(client: httpx.AsyncClient, auth) -> bool: