[Pydantic AI] Support MultiModalContent type in AgUI for tool_return data

### Description ```python case ToolMessage() as tool_msg: tool_call_id = tool_msg.tool_call_id tool_name = tool_calls.get(tool_call_id) if tool_name is None: # pragma: no cover raise ValueError(f'Tool call with ID {tool_call_id} not found in the history.') if tool_call_id.startswith(BUILTIN_TOOL_CALL_ID_PREFIX): _, provider_name, original_id = tool_call_id.split('|', 2) content: Any = tool_msg.content if isinstance(content, str): try: content = json.loads(content) except json.JSONDecodeError: pass builder.add( BuiltinToolReturnPart( tool_name=tool_name, content=content, tool_call_id=original_id, provider_name=provider_name, ) ) else: builder.add( ToolReturnPart( tool_name=tool_name, content=tool_msg.content,