Use ConversationId instead of raw Uuids (#3282)

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.

codex-rs/protocol/src/message_history.rs

@@ -3,7 +3,7 @@ use serde::Serialize;
 
 #[derive(Serialize, Deserialize, Debug, Clone)]
 pub struct HistoryEntry {
-    pub session_id: String,
+    pub conversation_id: String,
     pub ts: u64,
     pub text: String,
 }