Use ConversationId instead of raw Uuids

[gpeal]

We're trying to migrate from session_id: Uuid to conversation_id: ConversationId. Not only does this give us more type safety but it unifies our terminology across Codex and with the implementation of session resuming, a conversation (which can span multiple sessions) is more appropriate.

I started this impl on https://github.com/openai/codex/pull/3219 as part of getting resume working in the extension but it's big enough that it should be broken out.