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/in_memory.rs

@@ -39,7 +39,6 @@ use crate::ThreadStoreFuture;
 use crate::ThreadStoreResult;
 use crate::UpdateThreadMetadataParams;
 use crate::error::reject_paginated_history_mode;
-use crate::types::canonical_history_mode_from_rollout_items;
 
 static IN_MEMORY_THREAD_STORES: OnceLock<Mutex<HashMap<String, Arc<InMemoryThreadStore>>>> =
     OnceLock::new();
@@ -200,7 +199,7 @@ mod tests {
     }
 
     #[tokio::test]
-    async fn paginated_threads_allow_metadata_reads_and_reject_legacy_history_paths() {
+    async fn paginated_threads_allow_metadata_reads_and_resume_but_reject_legacy_history_paths() {
         let store = InMemoryThreadStore::default();
         let thread_id = ThreadId::default();
         let rollout_path = PathBuf::from("/tmp/paginated-thread.jsonl");
@@ -287,18 +286,16 @@ mod tests {
                 .await
                 .expect_err("history load should fail"),
         );
-        assert_paginated_threads_unsupported(
-            store
-                .resume_thread(ResumeThreadParams {
-                    thread_id,
-                    rollout_path: None,
-                    history: None,
-                    include_archived: false,
-                    metadata: thread_metadata(),
-                })
-                .await
-                .expect_err("resume should fail"),
-        );
+        store
+            .resume_thread(ResumeThreadParams {
+                thread_id,
+                rollout_path: None,
+                history: None,
+                include_archived: false,
+                metadata: thread_metadata(),
+            })
+            .await
+            .expect("resume should succeed");
         assert_paginated_threads_unsupported(
             store
                 .create_thread(create_thread_params(
@@ -463,13 +460,6 @@ impl InMemoryThreadStore {
     async fn resume_thread(&self, params: ResumeThreadParams) -> ThreadStoreResult<()> {
         let mut state = self.state.lock().await;
         state.calls.resume_thread += 1;
-        let history_mode = params
-            .history
-            .as_deref()
-            .map(Vec::as_slice)
-            .map(canonical_history_mode_from_rollout_items)
-            .unwrap_or_else(|| history_mode_from_state(&state, params.thread_id));
-        reject_paginated_history_mode(history_mode)?;
         if let Some(history) = params.history {
             state
                 .histories