Changing method in MCP notifications (#1684)

- Changing the codex/event type

codex-rs/core/src/protocol.rs

@@ -278,8 +278,9 @@ pub struct Event {
 }
 
 /// Response event from the agent
-#[derive(Debug, Clone, Deserialize, Serialize)]
+#[derive(Debug, Clone, Deserialize, Serialize, Display)]
 #[serde(tag = "type", rename_all = "snake_case")]
+#[strum(serialize_all = "lowercase")]
 pub enum EventMsg {
     /// Error while executing a submission
     Error(ErrorEvent),