refactor(search): address PR #750 round 8 review feedback

- Rename `verified_count` → `verified_chunk_count` to make the count
  granularity explicit at the field name (chunks vs unique docs).
- News verifier now fails open *per-item* on non-numeric stored doc_ids
  (matches notes/files/deck shape); a single bad id no longer rescues
  definitively-missing siblings from eviction.
- Update note-verifier integration test to use string doc_ids end-to-end
  to match production storage (scanner.py:241 stringifies note ids).
- Add regression test for the closed-task-group race guard in
  `verify_search_results` so the RuntimeError swallow is locked in.
- Convert remaining f-string logger calls in `server/semantic.py` to
  lazy %-style formatting (per repo convention).
- Document `evict_on_missing` as a developer/test flag (no env var) and
  flag the `get_file_info` 404→raise contract change in its docstring.
- Add a TODO(ADR-019) breadcrumb for the hardcoded 2× over-fetch so
  future tuning has a clear hook.

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

nextcloud_mcp_server/client/webdav.py

@@ -1301,6 +1301,15 @@ class WebDAVClient(BaseNextcloudClient):
     async def get_file_info(self, path: str) -> dict[str, Any] | None:
         """Get file info including file ID via WebDAV PROPFIND.
 
+        .. note::
+            **Behavior change (ADR-019):** previously this method returned
+            ``None`` for HTTP 404. It now raises ``HTTPStatusError`` for any
+            non-2xx status, including 404. ``None`` is reserved for the
+            ambiguous *malformed PROPFIND* case (server returned 2xx with a
+            response body missing required XML elements). External callers
+            updating from the old contract must catch ``HTTPStatusError``
+            and inspect ``e.response.status_code`` to handle 404 explicitly.
+
         Args:
             path: Path to the file (relative to user's files directory)