[codex] Migrate thread turns list to thread store (#19280)

- migrate `thread/turns/list` to ThreadStore. Uses ThreadStore for most
data now but merges in the in-memory state from thread manager
- keep v2 `thread/list` pathless-store friendly by converting
`StoredThread` directly to API `Thread`
- add regression coverage for pathless store history/listing

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

@@ -5,7 +5,6 @@
 //! any other backing store.
 
 mod error;
-#[cfg(debug_assertions)]
 mod in_memory;
 mod live_thread;
 mod local;
@@ -15,9 +14,7 @@ mod types;
 
 pub use error::ThreadStoreError;
 pub use error::ThreadStoreResult;
-#[cfg(debug_assertions)]
 pub use in_memory::InMemoryThreadStore;
-#[cfg(debug_assertions)]
 pub use in_memory::InMemoryThreadStoreCalls;
 pub use live_thread::LiveThread;
 pub use live_thread::LiveThreadInitGuard;