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/analytics/src/client_tests.rs

@@ -479,7 +479,7 @@ fn sample_thread(thread_id: &str) -> Thread {
         parent_thread_id: None,
         preview: "first prompt".to_string(),
         ephemeral: false,
-        is_pinned: false,
+        section: None,
         history_mode: Default::default(),
         model_provider: "openai".to_string(),
         created_at: 1,