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/exec/src/lib.rs

@@ -1485,7 +1485,7 @@ async fn resolve_resume_thread_id(
                         model_providers: model_providers.clone(),
                         source_kinds: Some(all_thread_source_kinds()),
                         archived: Some(false),
-                        is_pinned: None,
+                        section_id: None,
                         parent_thread_id: None,
                         ancestor_thread_id: None,
                         cwd: None,
@@ -1553,7 +1553,7 @@ async fn resolve_resume_thread_id(
                     model_providers: model_providers.clone(),
                     source_kinds: Some(all_thread_source_kinds()),
                     archived: Some(false),
-                    is_pinned: None,
+                    section_id: None,
                     parent_thread_id: None,
                     ancestor_thread_id: None,
                     cwd: None,

codex-rs/exec/src/lib_tests.rs

@@ -335,7 +335,7 @@ fn turn_items_for_thread_returns_matching_turn_items() {
         parent_thread_id: None,
         preview: String::new(),
         ephemeral: false,
-        is_pinned: false,
+        section: None,
         history_mode: Default::default(),
         model_provider: "openai".to_string(),
         created_at: 0,
@@ -803,7 +803,7 @@ fn sample_thread_start_response() -> ThreadStartResponse {
             parent_thread_id: None,
             preview: String::new(),
             ephemeral: false,
-            is_pinned: false,
+            section: None,
             history_mode: Default::default(),
             model_provider: "openai".to_string(),
             created_at: 0,