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/app-server/src/message_processor.rs

@@ -1114,6 +1114,9 @@ impl MessageProcessor {
             ClientRequest::ThreadMetadataUpdate { params, .. } => {
                 self.thread_processor.thread_metadata_update(params).await
             }
+            ClientRequest::ThreadSectionList { params, .. } => {
+                self.thread_processor.thread_section_list(params).await
+            }
             ClientRequest::ThreadSettingsUpdate { params, .. } => {
                 self.turn_processor
                     .thread_settings_update(&request_id, params)