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.rs

@@ -361,7 +361,7 @@ pub async fn list_threads_db(
     cwd_filters: Option<&[PathBuf]>,
     relation_filter: Option<codex_state::ThreadRelationFilter>,
     archived: bool,
-    is_pinned: Option<bool>,
+    section: Option<Option<&str>>,
     search_term: Option<&str>,
 ) -> Option<codex_state::ThreadsPage> {
     let ctx = context?;
@@ -403,7 +403,7 @@ pub async fn list_threads_db(
     if let Some(relation_filter) = relation_filter {
         let filters = codex_state::ThreadFilterOptions {
             archived_only: archived,
-            is_pinned,
+            section,
             allowed_sources: allowed_sources.as_slice(),
             model_providers: model_providers.as_deref(),
             cwd_filters: normalized_cwd_filters.as_deref(),
@@ -430,7 +430,7 @@ pub async fn list_threads_db(
     while items.len() < page_size {
         let filters = codex_state::ThreadFilterOptions {
             archived_only: archived,
-            is_pinned,
+            section,
             allowed_sources: allowed_sources.as_slice(),
             model_providers: model_providers.as_deref(),
             cwd_filters: normalized_cwd_filters.as_deref(),