fix(realtime): add conversation item reference type

[majiayu000]

Summary

• add realtime conversation item type for item_reference
• include item_reference in ConversationItem and ConversationItemParam unions
• add regression test to validate item_reference matches ConversationItem

Testing

• PYTHONPATH=src pytest tests/test_realtime_conversation_item_reference.py -o addopts= -o filterwarnings= -p no:benchmark
• env -u http_proxy -u https_proxy -u HTTP_PROXY -u HTTPS_PROXY -u ALL_PROXY -u all_proxy PYTHONPATH=src pytest -o addopts= -o filterwarnings= -p no:benchmark (fails: tests/api_resources/test_videos.py expects model=string but Prism spec only allows sora-2/sora-2-pro, 422)