add multimodal_message WebSocket event

[sedatcagdas]

Summary

• Add support for the multimodal_message client-to-server WebSocket event, enabling users to send messages that combine text and file references (per the Agents WebSocket API)
• Define MultimodalMessageFile and MultimodalMessageClientToOrchestratorEvent classes in conversation.py, following the existing event class pattern
• Expose send_multimodal_message(text?, file_id?) on both Conversation (sync) and AsyncConversation (async)