Update docs to account for system_prompt in structured_output (#158)

Co-authored-by: Mackenzie Zastrow <[email protected]>

Author: zastrowm

docs/user-guide/concepts/model-providers/custom_model_provider.md

@@ -332,21 +332,26 @@ T = TypeVar('T', bound=BaseModel)
 
 @override
 async def structured_output(
-    self, output_model: Type[T], prompt: Messages, **kwargs: Any
+    self,
+    output_model: Type[T],
+    prompt: Messages,
+    system_prompt: Optional[str] = None,
+    **kwargs: Any
 ) -> Generator[dict[str, Union[T, Any]], None, None]:
     """Get structured output using tool calling.
 
     Args:
         output_model: The output model to use for the agent.
         prompt: The prompt messages to use for the agent.
+        system_prompt: The system prompt to use for the agent.
         **kwargs: Additional keyword arguments for future extensibility.
     """
 
     # Convert Pydantic model to tool specification
     tool_spec = convert_pydantic_to_tool_spec(output_model)
 
     # Use the stream method with tool specification
-    response = await self.stream(messages=prompt, tool_specs=[tool_spec], **kwargs)
+    response = await self.stream(messages=prompt, tool_specs=[tool_spec], system_prompt=system_prompt, **kwargs)
 
     # Process streaming response
     async for event in process_stream(response, prompt):