app-server: Add ephemeral field to Thread object (#13084)

Currently there is no alternative way to know that thread is ephemeral,
only client which did create it has the knowledge.

codex-rs/app-server/src/thread_status.rs

@@ -733,6 +733,7 @@ mod tests {
         Thread {
             id: thread_id.to_string(),
             preview: String::new(),
+            ephemeral: false,
             model_provider: "mock-provider".to_string(),
             created_at: 0,
             updated_at: 0,