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.