[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/in_memory.rs

@@ -79,7 +79,7 @@ mod tests {
         let items_err = store
             .list_items(ListItemsParams {
                 thread_id,
-                turn_id: "turn_1".to_string(),
+                turn_id: None,
                 include_archived: true,
                 cursor: None,
                 page_size: 10,