fix: Preserve system_instruction in LangchainAgent clone()

COPYBARA_INTEGRATE_REVIEW=#5047 from SaschaHeyer:fix-missing-system-instruction 28502c3 PiperOrigin-RevId: 736154906

Author: SaschaHeyer

vertexai/preview/reasoning_engines/templates/langchain.py

@@ -567,6 +567,7 @@ def clone(self) -> "LangchainAgent":
 
  return LangchainAgent(
  model=self._model_name,
+ system_instruction=self._system_instruction,
  prompt=copy.deepcopy(self._prompt),
  tools=copy.deepcopy(self._tools),
  output_parser=copy.deepcopy(self._output_parser),