[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/app-server/tests/suite/v2/mod.rs

@@ -34,6 +34,8 @@ mod plugin_read;
 mod plugin_uninstall;
 mod rate_limits;
 mod realtime_conversation;
+#[cfg(debug_assertions)]
+mod remote_thread_store;
 mod request_permissions;
 mod request_user_input;
 mod review;