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>

tests/test_tools_filestation.py

@@ -1353,7 +1353,7 @@ async def test_extract_success(config: AppConfig) -> None:
     assert start_call[0][1] == "start"
     assert start_call[1]["version"] == 2
     p = start_call[1]["params"]
-    assert json.loads(p["path"]) == "/backup/archive.zip"
+    assert json.loads(p["file_path"]) == "/backup/archive.zip"
     assert json.loads(p["dest_folder_path"]) == "/data/extracted"
     assert p["overwrite"] == "false"
     assert p["keep_dir"] == "true"