langchain[patch]: Invoke callback prior to yielding token (#18282)

## PR title
langchain[patch]: Invoke callback prior to yielding

## PR message
Description: Invoke on_llm_new_token callback prior to yielding token in
_stream and _astream methods in langchain/tests/fake_chat_model.
Issue: https://github.com/langchain-ai/langchain/issues/16913
Dependencies: None
Twitter handle: None