google-genai[patch]: Invoke callback prior to yielding token (#17092)

- **Description:** Invoke callback prior to yielding token in stream and
astream methods for Google-genai,
  - **Issue:** the issue # 16913,
  - **Twitter handle:** Sparsh10649446

---------

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

libs/partners/google-genai/langchain_google_genai/chat_models.py

@@ -624,9 +624,9 @@ class ChatGoogleGenerativeAI(_BaseGoogleGenerativeAI, BaseChatModel):
                 generation_t=ChatGenerationChunk,
             )
             gen = cast(ChatGenerationChunk, _chat_result.generations[0])
-            yield gen
             if run_manager:
                 run_manager.on_llm_new_token(gen.text)
+            yield gen
 
     async def _astream(
         self,
@@ -654,9 +654,9 @@ class ChatGoogleGenerativeAI(_BaseGoogleGenerativeAI, BaseChatModel):
                 generation_t=ChatGenerationChunk,
             )
             gen = cast(ChatGenerationChunk, _chat_result.generations[0])
-            yield gen
             if run_manager:
                 await run_manager.on_llm_new_token(gen.text)
+            yield gen
 
     def _prepare_chat(
         self,