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/core/src/error.rs

@@ -1,10 +1,10 @@
+use codex_protocol::mcp_protocol::ConversationId;
 use reqwest::StatusCode;
 use serde_json;
 use std::io;
 use std::time::Duration;
 use thiserror::Error;
 use tokio::task::JoinError;
-use uuid::Uuid;
 
 pub type Result<T> = std::result::Result<T, CodexErr>;
 
@@ -49,7 +49,7 @@ pub enum CodexErr {
     Stream(String, Option<Duration>),
 
     #[error("no conversation with id: {0}")]
-    ConversationNotFound(Uuid),
+    ConversationNotFound(ConversationId),
 
     #[error("session configured event was not the first event in the stream")]
     SessionConfiguredNotFirstEvent,