Add tool_calls field and make content optional in Message model

- Add optional tool_calls field to Message model
- Make content field optional with default None
- Allows assistant messages with tool_calls instead of content
- Fixes 422 validation errors for tool call messages
- Supports OpenAI message format with function calls

aisbf/models.py

@@ -28,7 +28,8 @@ from typing import Dict, List, Optional, Union
 
 class Message(BaseModel):
     role: str
-    content: Union[str, List[Dict], List]
+    content: Union[str, List[Dict], List, None] = None
+    tool_calls: Optional[List[Dict]] = None
 
 class ChatCompletionRequest(BaseModel):
     model: str