fix: retain last non-empty result set in search polling loop

DSM sends files=[] on the final finished=True poll even when results
exist in earlier rounds. Overwriting files each iteration discarded
the real results, producing a false "No files found" response.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/mcp_synology_filestation/tools/filestation.py

@@ -287,7 +287,9 @@ def register_filestation(
                     )
                 return f"Error: {e}"
 
-            files = poll_data.get("files", [])
+            current_files: list[dict] = poll_data.get("files", [])
+            if current_files:
+                files = current_files  # keep the last non-empty result set
             finished: bool = poll_data.get("finished", False)
 
             if finished: