[codex] Make thread store turn filter optional (#28949)

Make `ListItemsParams::turn_id` optional so callers can list persisted
items across an entire thread or narrow the result to one turn. This
aligns the thread-store API and documentation with thread-wide item
listing while preserving the optional turn-filter behavior for
implementations.

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

@@ -102,7 +102,7 @@ pub trait ThreadStore: Any + Send + Sync {
         })
     }
 
-    /// Lists persisted items within a stored turn.
+    /// Lists persisted items within a stored thread, optionally filtered to a turn.
     fn list_items(&self, _params: ListItemsParams) -> ThreadStoreFuture<'_, ItemPage> {
         Box::pin(async {
             Err(ThreadStoreError::Unsupported {