fix: search empty results + extract error 408 (v0.3.3)

Bug 1 — search::start folder_path format (already fixed in 314fae9):
  json.dumps([path]) is confirmed correct per official Synology API docs
  and multiple independent implementations (N4S4/synology-api, kwent/syno).
  Poll-loop last-non-empty guard (if current_files:) is also in place.
  No further change needed for Bug 1.

Bug 2 — extract::start wrong parameter key:
  The previous fix attempt renamed "file_path" → "path", which was wrong.
  Official API docs and independent implementations confirm the key is
  "file_path". The json.dumps() wrapping on file_path and dest_folder_path
  was already correct. Reverted the key rename.

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

src/mcp_synology_filestation/tools/filestation.py

@@ -845,7 +845,7 @@ def register_filestation(
                 "start",
                 version=2,
                 params={
-                    "path": json.dumps(file_path),
+                    "file_path": json.dumps(file_path),
                     "dest_folder_path": json.dumps(dest_folder_path),
                     "overwrite": "true" if overwrite else "false",
                     "keep_dir": "true" if keep_dir else "false",