chore: handle all cases for EventMsg (#936)

For now, this removes the `#[non_exhaustive]` directive on `EventMsg` so
that we are forced to handle all `EventMsg` by default. (We may revisit
this if/when we publish `core/` as a `lib` crate.) For now, it is
helpful to have this as a forcing function because we have effectively
two UIs (`tui` and `exec`) and usually when we add a new variant to
`EventMsg`, we want to be sure that we update both.

codex-rs/core/src/protocol.rs

@@ -298,7 +298,6 @@ pub struct Event {
 }
 
 /// Response event from the agent
-#[non_exhaustive]
 #[derive(Debug, Clone, Deserialize, Serialize)]
 #[serde(tag = "type", rename_all = "snake_case")]
 pub enum EventMsg {