mistralai: Fixed streaming in MistralAI with ainvoke and callbacks (#22000)

# Fix streaming in mistral with ainvoke 
- [x] **PR title**
- [x] **PR message**
- [x] **Add tests and docs**:
  1. [x] Added a test for the fixed integration.
2. [x] An example notebook showing its use. It lives in
`docs/docs/integrations` directory.
- [x] **Lint and test**: Ran `make format`, `make lint` and `make test`
from the root of the package(s) I've modified.

Hello 

* I Identified an issue in the mistral package where the callback
streaming (see on_llm_new_token) was not functioning correctly when the
streaming parameter was set to True and call with `ainvoke`.
* The root cause of the problem was the streaming not taking into
account. ( I think it's an oversight )
* To resolve the issue, I added the `streaming` attribut.
* Now, the callback with streaming works as expected when the streaming
parameter is set to True.

## How to reproduce

```
from langchain_mistralai.chat_models import ChatMistralAI
chain = ChatMistralAI(streaming=True)
# Add a callback
chain.ainvoke(..)

# Oberve on_llm_new_token
# Now, the callback is given as streaming tokens, before it was in grouped format.
```

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

libs/partners/mistralai/langchain_mistralai/chat_models.py

@@ -586,7 +586,7 @@ class ChatMistralAI(BaseChatModel):
         stream: Optional[bool] = None,
         **kwargs: Any,
     ) -> ChatResult:
-        should_stream = stream if stream is not None else False
+        should_stream = stream if stream is not None else self.streaming
         if should_stream:
             stream_iter = self._astream(
                 messages=messages, stop=stop, run_manager=run_manager, **kwargs