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/state/src/extract.rs

@@ -695,7 +695,7 @@ mod tests {
             tokens_used: 1,
             first_user_message: None,
             archived_at: None,
-            is_pinned: false,
+            section: None,
             git_sha: None,
             git_branch: None,
             git_origin_url: None,