[codex] Move persistence policy application into ThreadStore (#27318)

Move the application of the persistence policy into the thread store, so
thread stores can get raw append items rather than canonical append
items. This will enable store-specific projections over the raw input
items.

codex-rs/thread-store/src/lib.rs

@@ -40,6 +40,7 @@ pub use types::SearchThreadsParams;
 pub use types::SortDirection;
 pub use types::StoredThread;
 pub use types::StoredThreadHistory;
+pub use types::StoredThreadItem;
 pub use types::StoredThreadSearchResult;
 pub use types::StoredTurn;
 pub use types::StoredTurnError;