[codex] add non-local thread store regression harness (#19266)

- Add an integration test that guarantees nothing gets written to codex
home dir or sqlite when running a rollout with a non-local ThreadStore
- Add an in-memory "spy" ThreadStore for tests like this

Note I could not find a good way to also ensure there were no filesystem
_reads_ that didn't go through threadstore. I explored a more elaborate
sandboxed-subprocess approach but it isn't platform portable and felt
like it wasn't (yet) worth it.

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

@@ -5,6 +5,8 @@
 //! any other backing store.
 
 mod error;
+#[cfg(debug_assertions)]
+mod in_memory;
 mod live_thread;
 mod local;
 mod remote;
@@ -13,6 +15,10 @@ 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;
 pub use local::LocalThreadStore;