Migrate fork and resume reads to thread store (#18900)

- Route cold thread/resume and thread/fork source loading through
ThreadStore reads instead of direct rollout path operations
- Keep lookups that explicitly specify a rollout-path using the local
thread store methods but return an invalid-request error for remote
ThreadStore configurations
- Add some additional unit tests for code path coverage

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

@@ -9,6 +9,7 @@ use crate::ArchiveThreadParams;
 use crate::CreateThreadParams;
 use crate::ListThreadsParams;
 use crate::LoadThreadHistoryParams;
+use crate::ReadThreadByRolloutPathParams;
 use crate::ReadThreadParams;
 use crate::ResumeThreadParams;
 use crate::StoredThread;
@@ -25,6 +26,10 @@ mod proto;
 
 /// gRPC-backed [`ThreadStore`] implementation for deployments whose durable thread data lives
 /// outside the app-server process.
+///
+/// This store is still a work in progress: app-server code should call the generic
+/// [`ThreadStore`] methods, and unsupported remote operations will return explicit
+/// `not_implemented` errors until the remote API catches up.
 #[derive(Clone, Debug)]
 pub struct RemoteThreadStore {
     endpoint: String,
@@ -187,6 +192,15 @@ impl ThreadStore for RemoteThreadStore {
         helpers::stored_thread_from_proto(thread)
     }
 
+    async fn read_thread_by_rollout_path(
+        &self,
+        _params: ReadThreadByRolloutPathParams,
+    ) -> ThreadStoreResult<StoredThread> {
+        Err(ThreadStoreError::Internal {
+            message: "remote thread store does not support read_thread_by_rollout_path".to_string(),
+        })
+    }
+
     async fn list_threads(&self, params: ListThreadsParams) -> ThreadStoreResult<ThreadPage> {
         list_threads::list_threads(self, params).await
     }