Read conversation summaries through thread store (#18716)

Migrate the conversation summary App Server methods to ThreadStore

Because this app server api allows explicitly fetching the thread by
rollout path, intercept that case in the app server code and (a) route
directly to underlying local thread store methods if we're using a local
thread store, or (b) throw an unsupported error if we're using a remote
thread store. This keeps the thread store API clean and all filesystem
operations inside of the local thread store, which pushing the
"fundamental incompatibility" check as early as possible.

codex-rs/thread-store/src/remote/mod.rs

@@ -48,6 +48,10 @@ impl RemoteThreadStore {
 
 #[async_trait]
 impl ThreadStore for RemoteThreadStore {
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+
     async fn create_thread(
         &self,
         _params: CreateThreadParams,