codex: move unloaded thread writes into store (#18361)

- Migrates unloaded `thread/name/set` and `thread/memoryModeSet`
app-server writes behind the generic
`ThreadStore::update_thread_metadata` API rather than adding one-off
store methods for setting thread name or memory mode.
- Implements the local ThreadStore metadata patch path for thread name
and memory mode, including rollout append, legacy name index updates,
SessionMeta validation/update, SQLite reconciliation, and re-reading the
stored thread.
- Adds focused local thread-store unit coverage plus app-server
integration coverage for the migrated unloaded write paths.

codex-rs/thread-store/src/remote/mod.rs

@@ -10,7 +10,6 @@ use crate::ListThreadsParams;
 use crate::LoadThreadHistoryParams;
 use crate::ReadThreadParams;
 use crate::ResumeThreadRecorderParams;
-use crate::SetThreadNameParams;
 use crate::StoredThread;
 use crate::StoredThreadHistory;
 use crate::ThreadPage;
@@ -82,10 +81,6 @@ impl ThreadStore for RemoteThreadStore {
         list_threads::list_threads(self, params).await
     }
 
-    async fn set_thread_name(&self, _params: SetThreadNameParams) -> ThreadStoreResult<()> {
-        Err(not_implemented("set_thread_name"))
-    }
-
     async fn update_thread_metadata(
         &self,
         _params: UpdateThreadMetadataParams,