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/app-server/src/thread_state.rs

@@ -40,6 +40,7 @@ pub(crate) struct PendingThreadResumeRequest {
     pub(crate) include_turns: bool,
     pub(crate) initial_turns_page:
         Option<codex_app_server_protocol::ThreadResumeInitialTurnsPageParams>,
+    pub(crate) resume_cursor_store: Option<Arc<dyn codex_thread_store::ThreadStore>>,
     pub(crate) redact_resume_payloads: bool,
 }