chore(app-server): mark thread/rollback as deprecated (#29928)

We will drop support for this in the near future due to the complexity
it introduces.

codex-rs/app-server-protocol/src/protocol/v2/thread.rs

@@ -1025,6 +1025,7 @@ pub struct ThreadBackgroundTerminalsTerminateResponse {
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
+/// DEPRECATED: `thread/rollback` will be removed soon.
 pub struct ThreadRollbackParams {
     pub thread_id: String,
     /// The number of turns to drop from the end of the thread. Must be >= 1.