Add app-server support for reverting paginated threads (#38440)

## What changed

- Add the experimental `thread/revert` request, which replaces a loaded paginated thread's durable history with the prefix before `beforeTurnId` while preserving the thread ID.
- Interrupt any active turn, reload the replacement history without unloading the thread, preserve mutable thread settings, and emit `thread/reverted`.
- Return backward pagination cursors for the retained turns and items, reject stale rollout paths after a revert, and document that local file changes are unaffected.

## Testing

- Cover history replacement, pagination, stale paths, missing turns, subsequent turns, active-turn interruption, and preserved thread settings.

GitOrigin-RevId: 435a8163ad4f310cc47219b436f6bdde21f2b83a

codex-rs/app-server/src/message_processor.rs

@@ -1181,6 +1181,16 @@ impl MessageProcessor {
                     .thread_rollback(&request_id, params, app_server_client_name.as_deref())
                     .await
             }
+            ClientRequest::ThreadRevert { params, .. } => {
+                self.thread_processor
+                    .thread_revert(
+                        request_id.clone(),
+                        params,
+                        app_server_client_name.clone(),
+                        client_version.clone(),
+                    )
+                    .await
+            }
             ClientRequest::ThreadList { params, .. } => {
                 self.thread_processor.thread_list(params).await
             }