partners: fix default value for stop_sequences in ChatGroq (#28924)

- **Description:**  
This PR addresses an issue with the `stop_sequences` field in the
`ChatGroq` class. Currently, the field is defined as:
```python
stop: Optional[Union[List[str], str]] = Field(None, alias="stop_sequences")
```  
This causes the language server (LSP) to raise an error indicating that
the `stop_sequences` parameter must be implemented. The issue occurs
because `Field(None, alias="stop_sequences")` is different compared to
`Field(default=None, alias="stop_sequences")`.


![image](https://github.com/user-attachments/assets/bfc34cb1-c664-4c31-b856-8f18419c7350)
To resolve the issue, the field is updated to:  
```python
stop: Optional[Union[List[str], str]] = Field(default=None, alias="stop_sequences")
```  
While this issue does not affect runtime behavior, it ensures
compatibility with LSPs and improves the development experience.
- **Issue:** N/A  
- **Dependencies:** None

libs/partners/groq/langchain_groq/chat_models.py

@@ -305,7 +305,7 @@ class ChatGroq(BaseChatModel):
     """Model name to use."""
     temperature: float = 0.7
     """What sampling temperature to use."""
-    stop: Optional[Union[List[str], str]] = Field(None, alias="stop_sequences")
+    stop: Optional[Union[List[str], str]] = Field(default=None, alias="stop_sequences")
     """Default stop sequences."""
     model_kwargs: Dict[str, Any] = Field(default_factory=dict)
     """Holds any model parameters valid for `create` call not explicitly specified."""