Migrate archive/unarchive to local ThreadStore (#17892)

# Summary
- implement local ThreadStore archive/unarchive operations
- implement local ThreadStore read_thread operation
- break up the various ThreadStore local method implementations into
separate files
- migrate app-server archive/unarchive and core archive fixture to use
ThreadStore (but not all read operations yet!)
- use the ThreadStore's read operation as a proxy check for thread
persistence/existence in the app server code
- move all other filesystem operations related to archive (path
validation etc) into the local thread store.

# Tests
- add dedicated local store archive/unarchive tests

codex-rs/rollout/src/lib.rs

@@ -48,6 +48,7 @@ pub use list::get_threads_in_root;
 pub use list::parse_cursor;
 pub use list::read_head_for_summary;
 pub use list::read_session_meta_line;
+pub use list::read_thread_item_from_rollout;
 pub use list::rollout_date_parts;
 pub use metadata::builder_from_items;
 pub use policy::EventPersistenceMode;

codex-rs/rollout/src/list.rs

@@ -756,6 +756,21 @@ async fn build_thread_item(
     None
 }
 
+/// Read a single rollout file into the same summary item shape used by thread listing.
+///
+/// This is for callers that already resolved a rollout path and need the same
+/// metadata/preview extraction as list operations without scanning the whole
+/// sessions tree.
+pub async fn read_thread_item_from_rollout(path: PathBuf) -> Option<ThreadItem> {
+    build_thread_item(
+        path,
+        &[],
+        /*provider_matcher*/ None,
+        /*updated_at*/ None,
+    )
+    .await
+}
+
 /// Collects immediate subdirectories of `parent`, parses their (string) names with `parse`,
 /// and returns them sorted descending by the parsed key.
 async fn collect_dirs_desc<T, F>(parent: &Path, parse: F) -> io::Result<Vec<(T, PathBuf)>>