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-transport/src/outgoing_message.rs

@@ -21,6 +21,7 @@ impl fmt::Display for ConnectionId {
 /// Outgoing message from the server to the client.
 #[derive(Debug, Clone, Serialize)]
 #[serde(untagged)]
+#[allow(clippy::large_enum_variant)]
 pub enum OutgoingMessage {
     Request(ServerRequest),
     /// AppServerNotification is specific to the case where this is run as an