google-vertexai[patch]: Fixed SafetySettings handling in streaming API in VertexAI (#17278)

The streaming API doesn't separate safety_settings from the
generation_config payload. As the result the following error is observed
when using `stream` API. The functionality is correct with `invoke` API.

The fix separates the `safety_settings` from params and sets it as
argument to the `send_message` method.

```
ERROR:         Unknown field for GenerationConfig: safety_settings
Traceback (most recent call last):
  File "/Users/user/Library/Caches/pypoetry/virtualenvs/chatbot-worker-main-Ju-qIM-X-py3.12/lib/python3.12/site-packages/langchain_core/language_models/chat_models.py", line 250, in stream
    raise e
  File "/Users/user/Library/Caches/pypoetry/virtualenvs/chatbot-worker-main-Ju-qIM-X-py3.12/lib/python3.12/site-packages/langchain_core/language_models/chat_models.py", line 234, in stream
    for chunk in self._stream(
  File "/Users/user/Library/Caches/pypoetry/virtualenvs/chatbot-worker-main-Ju-qIM-X-py3.12/lib/python3.12/site-packages/langchain_google_vertexai/chat_models.py", line 501, in _stream
    for response in responses:
  File "/Users/user/Library/Caches/pypoetry/virtualenvs/chatbot-worker-main-Ju-qIM-X-py3.12/lib/python3.12/site-packages/vertexai/generative_models/_generative_models.py", line 921, in _send_message_streaming
    for chunk in stream:
  File "/Users/user/Library/Caches/pypoetry/virtualenvs/chatbot-worker-main-Ju-qIM-X-py3.12/lib/python3.12/site-packages/vertexai/generative_models/_generative_models.py", line 514, in _generate_content_streaming
    request = self._prepare_request(
              ^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/user/Library/Caches/pypoetry/virtualenvs/chatbot-worker-main-Ju-qIM-X-py3.12/lib/python3.12/site-packages/vertexai/generative_models/_generative_models.py", line 256, in _prepare_request
    gapic_generation_config = gapic_content_types.GenerationConfig(
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/user/Library/Caches/pypoetry/virtualenvs/chatbot-worker-main-Ju-qIM-X-py3.12/lib/python3.12/site-packages/proto/message.py", line 576, in __init__
    raise ValueError(
ValueError: Unknown field for GenerationConfig: safety_settings
```

---------

Co-authored-by: Erick Friis <erick@langchain.dev>

libs/partners/google-vertexai/langchain_google_vertexai/chat_models.py

@@ -510,8 +510,13 @@ class ChatVertexAI(_VertexAICommon, BaseChatModel):
             # set param to `functions` until core tool/function calling implemented
             raw_tools = params.pop("functions") if "functions" in params else None
             tools = _format_tools_to_vertex_tool(raw_tools) if raw_tools else None
+            safety_settings = params.pop("safety_settings", None)
             responses = chat.send_message(
-                message, stream=True, generation_config=params, tools=tools
+                message,
+                stream=True,
+                generation_config=params,
+                safety_settings=safety_settings,
+                tools=tools,
             )
             for response in responses:
                 message = _parse_response_candidate(response.candidates[0])