refactor(search): address PR #750 round 10 review feedback

- Tighten verify_search_results signature: client: Any → NextcloudClientProtocol
- Collapse 3 copy-pasted lock-justification comments to a single-line pointer
- Add logger.debug timing around the verify_search_results call site
- Add logger.debug timing around the unbounded news.get_items fetch
- Rename SemanticSearchResponse.dropped_count → dropped_document_count to make
  the chunks-vs-documents unit asymmetry explicit at the API boundary
- Drop unreachable duplicate 409 branch in WebDAVClient.move_resource

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

nextcloud_mcp_server/client/webdav.py

@@ -483,13 +483,6 @@ class WebDAVClient(BaseNextcloudClient):
                     "status_code": 409,
                     "message": "Parent directory of destination doesn't exist",
                 }
-                logger.debug(
-                    f"Parent directory of destination '{destination_path}' doesn't exist"
-                )
-                return {
-                    "status_code": 409,
-                    "message": "Parent directory of destination doesn't exist",
-                }
             else:
                 logger.error(
                     f"HTTP error moving resource from '{source_path}' to '{destination_path}': {e}"