[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/in_memory.rs

@@ -61,9 +61,9 @@ pub struct InMemoryThreadStoreCalls {
     pub unarchive_thread: usize,
 }
 
-/// Test-only in-memory [`ThreadStore`] implementation.
+/// In-memory [`ThreadStore`] implementation for tests and debug configs.
 ///
-/// Debug/test configs can select this store by id, letting tests exercise
+/// Test and debug configs can select this store by id, letting tests exercise
 /// config-driven non-local persistence without requiring the real remote gRPC
 /// service.
 #[derive(Default)]