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.rs

@@ -104,6 +104,7 @@ use tracing::instrument;
 use tracing::trace;
 use tracing::trace_span;
 use tracing::warn;
+use uuid::Uuid;
 
 use crate::ModelProviderInfo;
 use crate::client::ModelClient;
@@ -248,7 +249,6 @@ use codex_utils_readiness::ReadinessFlag;
 /// The high-level interface to the Codex system.
 /// It operates as a queue pair where you send submissions and receive events.
 pub struct Codex {
-    pub(crate) next_id: AtomicU64,
     pub(crate) tx_sub: Sender<Submission>,
     pub(crate) rx_event: Receiver<Event>,
     // Last known status of the agent.
@@ -429,7 +429,6 @@ impl Codex {
             submission_loop(Arc::clone(&session), config, rx_sub).instrument(session_loop_span),
         );
         let codex = Codex {
-            next_id: AtomicU64::new(0),
             tx_sub,
             rx_event,
             agent_status: agent_status_rx,
@@ -446,10 +445,7 @@ impl Codex {
 
     /// Submit the `op` wrapped in a `Submission` with a unique ID.
     pub async fn submit(&self, op: Op) -> CodexResult<String> {
-        let id = self
-            .next_id
-            .fetch_add(1, std::sync::atomic::Ordering::SeqCst)
-            .to_string();
+        let id = Uuid::now_v7().to_string();
         let sub = Submission { id: id.clone(), op };
         self.submit_with_id(sub).await?;
         Ok(id)
@@ -3869,6 +3865,7 @@ pub(crate) async fn run_turn(
     let total_usage_tokens = sess.get_total_token_usage().await;
 
     let event = EventMsg::TurnStarted(TurnStartedEvent {
+        turn_id: turn_context.sub_id.clone(),
         model_context_window: turn_context.model_context_window(),
         collaboration_mode_kind: turn_context.collaboration_mode.mode,
     });
@@ -4822,6 +4819,7 @@ async fn try_run_sampling_request(
 ) -> CodexResult<SamplingRequestResult> {
     let collaboration_mode = sess.current_collaboration_mode().await;
     let rollout_item = RolloutItem::TurnContext(TurnContextItem {
+        turn_id: Some(turn_context.sub_id.clone()),
         cwd: turn_context.cwd.clone(),
         approval_policy: turn_context.approval_policy,
         sandbox_policy: turn_context.sandbox_policy.clone(),