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Add `thread/rollback` to app-server to support IDEs undo-ing the last N turns of a thread. For context, an IDE partner will be supporting an "undo" capability where the IDE (the app-server client) will be responsible for reverting the local changes made during the last turn. To support this well, we also need a way to drop the last turn (or more generally, the last N turns) from the agent's context. This is what `thread/rollback` does. **Core idea**: A Thread rollback is represented as a persisted event message (EventMsg::ThreadRollback) in the rollout JSONL file, not by rewriting history. On resume, both the model's context (core replay) and the UI turn list (app-server v2's thread history builder) apply these markers so the pruned history is consistent across live conversations and `thread/resume`. Implementation notes: - Rollback only affects agent context and appends to the rollout file; clients are responsible for reverting files on disk. - If a thread rollback is currently in progress, subsequent `thread/rollback` calls are rejected. - Because we use `CodexConversation::submit` and codex core tracks active turns, returning an error on concurrent rollbacks is communicated via an `EventMsg::Error` with a new variant `CodexErrorInfo::ThreadRollbackFailed`. app-server watches for that and sends the BAD_REQUEST RPC response. Tests cover thread rollbacks in both core and app-server, including when `num_turns` > existing turns (which clears all turns). **Note**: this explicitly does **not** behave like `/undo` which we just removed from the CLI, which does the opposite of what `thread/rollback` does. `/undo` reverts local changes via ghost commits/snapshots and does not modify the agent's context / conversation history.