Add persisted sections for organizing threads (#35722)

## What changed

- Replace the `isPinned` thread metadata and filters with an optional persisted
  `section` and `sectionId`.
- Add the paginated `threadSection/list` app-server method so clients can
  discover sections even when they contain no threads.
- Seed a stable `Pinned` section, validate section assignments, and support
  filtering for a specific section or for unsectioned threads.

## Testing

- Cover section protocol serialization, listing and pagination, metadata
  updates, filtering, persistence, migration compatibility, and operation
  without SQLite state.

GitOrigin-RevId: 7972b5471d29317b9387bfd90aa9f573f691ad4c

codex-rs/rollout/src/state_db_tests.rs

@@ -83,7 +83,7 @@ async fn list_threads_db_rejects_mismatched_sqlite_config_without_cleanup() -> a
         /*cwd_filters*/ None,
         /*relation_filter*/ None,
         /*archived*/ false,
-        /*is_pinned*/ None,
+        /*section*/ None,
         /*search_term*/ None,
     )
     .await;