[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/src/codex_message_processor.rs

@@ -235,6 +235,7 @@ use codex_core::clear_memory_roots_contents;
 use codex_core::config::Config;
 use codex_core::config::ConfigOverrides;
 use codex_core::config::NetworkProxyAuditMetadata;
+use codex_core::config::ThreadStoreConfig;
 use codex_core::config::edit::ConfigEdit;
 use codex_core::config::edit::ConfigEditsBuilder;
 use codex_core::config_loader::CloudRequirementsLoadError;
@@ -353,6 +354,8 @@ use codex_state::ThreadMetadata;
 use codex_state::ThreadMetadataBuilder;
 use codex_state::log_db::LogDbLayer;
 use codex_thread_store::ArchiveThreadParams as StoreArchiveThreadParams;
+#[cfg(debug_assertions)]
+use codex_thread_store::InMemoryThreadStore;
 use codex_thread_store::ListThreadsParams as StoreListThreadsParams;
 use codex_thread_store::LocalThreadStore;
 use codex_thread_store::ReadThreadByRolloutPathParams as StoreReadThreadByRolloutPathParams;
@@ -661,9 +664,11 @@ pub(crate) struct CodexMessageProcessorArgs {
 }
 
 fn configured_thread_store(config: &Config) -> Arc<dyn ThreadStore> {
-    match config.experimental_thread_store_endpoint.as_deref() {
-        Some(endpoint) => Arc::new(RemoteThreadStore::new(endpoint)),
-        None => Arc::new(configured_local_thread_store(config)),
+    match &config.experimental_thread_store {
+        ThreadStoreConfig::Local => Arc::new(configured_local_thread_store(config)),
+        ThreadStoreConfig::Remote { endpoint } => Arc::new(RemoteThreadStore::new(endpoint)),
+        #[cfg(debug_assertions)]
+        ThreadStoreConfig::InMemory { id } => InMemoryThreadStore::for_id(id),
     }
 }