fix: remove unsupported additional fields from list_dir

SYNO.FileStation.List::list returns error 408 ("Non-supported additional
field") for real_path, perm, and type. Reduce additional to ["size","time"]
— the only fields reliably supported across DSM versions. isdir is already
present in the default response.

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

src/mcp_synology_filestation/tools/filestation.py

@@ -151,7 +151,7 @@ def register_filestation(
                     "limit": limit,
                     "sort_by": sort_by,
                     "sort_direction": sort_direction,
-                    "additional": json.dumps(["real_path", "size", "time", "perm", "type"]),
+                    "additional": json.dumps(["size", "time"]),
                 },
             )
         except SynologyError as e: