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/mcp-server/tests/suite/list_resume.rs

@@ -142,7 +142,7 @@ async fn test_list_and_resume_conversations() {
     } = to_response::<ResumeConversationResponse>(resume_resp)
         .expect("deserialize resumeConversation response");
     // conversation id should be a valid UUID
-    let _ = uuid::Uuid::from_bytes(conversation_id.0.into_bytes());
+    let _: uuid::Uuid = conversation_id.into();
 }
 
 fn create_fake_rollout(codex_home: &Path, filename_ts: &str, meta_rfc3339: &str, preview: &str) {