chore: persist turn_id in rollout session and make turn_id uuid based (#11246)

Problem:
1. turn id is constructed in-memory;
2. on resuming threads, turn_id might not be unique;
3. client cannot no the boundary of a turn from rollout files easily.

This PR does three things:
1. persist `task_started` and `task_complete` events;
1. persist `turn_id` in rollout turn events;
5. generate turn_id as unique uuids instead of incrementing it in
memory.

This helps us resolve the issue of clients wanting to have unique turn
ids for resuming a thread, and knowing the boundry of each turn in
rollout files.

example debug logs
```
2026-02-11T00:32:10.746876Z DEBUG codex_app_server_protocol::protocol::thread_history: built turn from rollout items turn_index=8 turn=Turn { id: "019c4a07-d809-74c3-bc4b-fd9618487b4b", items: [UserMessage { id: "item-24", content: [Text { text: "hi", text_elements: [] }] }, AgentMessage { id: "item-25", text: "Hi. I’m in the workspace with your current changes loaded and ready. Send the next task and I’ll execute it end-to-end." }], status: Completed, error: None }
2026-02-11T00:32:10.746888Z DEBUG codex_app_server_protocol::protocol::thread_history: built turn from rollout items turn_index=9 turn=Turn { id: "019c4a18-1004-76c0-a0fb-a77610f6a9b8", items: [UserMessage { id: "item-26", content: [Text { text: "hello", text_elements: [] }] }, AgentMessage { id: "item-27", text: "Hello. Ready for the next change in `codex-rs`; I can continue from the current in-progress diff or start a new task." }], status: Completed, error: None }
2026-02-11T00:32:10.746899Z DEBUG codex_app_server_protocol::protocol::thread_history: built turn from rollout items turn_index=10 turn=Turn { id: "019c4a19-41f0-7db0-ad78-74f1503baeb8", items: [UserMessage { id: "item-28", content: [Text { text: "hello", text_elements: [] }] }, AgentMessage { id: "item-29", text: "Hello. Send the specific change you want in `codex-rs`, and I’ll implement it and run the required checks." }], status: Completed, error: None }
```

backward compatibility:
if you try to resume an old session without task_started and
task_complete event populated, the following happens:
- If you resume and do nothing: those reconstructed historical IDs can
differ next time you resume.
- If you resume and send a new turn: the new turn gets a fresh UUID from
live submission flow and is persisted, so that new turn’s ID is stable
on later resumes.
I think this behavior is fine, because we only care about deterministic
turn id once a turn is triggered.

codex-rs/core/src/codex_delegate.rs

@@ -1,6 +1,5 @@
 use std::collections::HashMap;
 use std::sync::Arc;
-use std::sync::atomic::AtomicU64;
 
 use async_channel::Receiver;
 use async_channel::Sender;
@@ -90,7 +89,6 @@ pub(crate) async fn run_codex_thread_interactive(
     });
 
     Ok(Codex {
-        next_id: AtomicU64::new(0),
         tx_sub: tx_ops,
         rx_event: rx_sub,
         agent_status: codex.agent_status.clone(),
@@ -166,7 +164,6 @@ pub(crate) async fn run_codex_thread_one_shot(
     drop(rx_closed);
 
     Ok(Codex {
-        next_id: AtomicU64::new(0),
         rx_event: rx_bridge,
         tx_sub: tx_closed,
         agent_status,
@@ -470,7 +467,6 @@ mod tests {
         let (_agent_status_tx, agent_status) = watch::channel(AgentStatus::PendingInit);
         let (session, ctx, _rx_evt) = crate::codex::make_session_and_context_with_rx().await;
         let codex = Arc::new(Codex {
-            next_id: AtomicU64::new(0),
             tx_sub,
             rx_event: rx_events,
             agent_status,
@@ -482,6 +478,7 @@ mod tests {
             .send(Event {
                 id: "full".to_string(),
                 msg: EventMsg::TurnAborted(TurnAbortedEvent {
+                    turn_id: Some("turn-1".to_string()),
                     reason: TurnAbortReason::Interrupted,
                 }),
             })