Enable paginated thread history in app-server (#33364)

## What changed

- Allow `thread/start` to select experimental `historyMode: "paginated"` when the thread store supports turn and item pagination.
- Resume paginated threads with bounded model context and require `excludeTurns: true` instead of loading full history.
- Return `turnsBackwardsCursor` and `itemsBackwardsCursor` from `thread/resume` so clients can hydrate durable history through `thread/turns/list` and `thread/items/list` while newer records arrive through live notifications.
- Reject operations that still require full history, including `initialTurnsPage`, `thread/read` with `includeTurns: true`, `thread/rollback`, and detached review.
- Make item cursors independent of the optional `turnId` filter.

## Testing

- Cover paginated thread start, cold and live resume, backwards cursor hydration, metadata reads, unsupported operations, and stores without pagination support.

GitOrigin-RevId: c42f9b268e29d049843330f474fa3ecde12f5b8a

codex-rs/thread-store/src/store.rs

@@ -105,6 +105,11 @@ pub trait ThreadStore: Any + Send + Sync {
     /// Lists stored threads matching the supplied filters.
     fn list_threads(&self, params: ListThreadsParams) -> ThreadStoreFuture<'_, ThreadPage>;
 
+    /// Whether paginated threads can hydrate durable history through turn and item lists.
+    fn supports_paginated_history_lists(&self) -> bool {
+        false
+    }
+
     /// Searches stored threads and returns search-only preview metadata.
     fn search_threads(
         &self,