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/store.rs

@@ -1,3 +1,5 @@
+use std::any::Any;
+
 use async_trait::async_trait;
 
 use crate::AppendThreadItemsParams;
@@ -16,7 +18,11 @@ use crate::UpdateThreadMetadataParams;
 
 /// Storage-neutral thread persistence boundary.
 #[async_trait]
-pub trait ThreadStore: Send + Sync {
+pub trait ThreadStore: Any + Send + Sync {
+    /// Return this store as [`Any`] so callers at API boundaries can reject requests that only
+    /// make sense for a concrete store implementation.
+    fn as_any(&self) -> &dyn Any;
+
     /// Creates a new thread and returns a live recorder for future appends.
     async fn create_thread(
         &self,