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/exec/tests/event_processor_with_json_output.rs

@@ -117,6 +117,7 @@ fn task_started_produces_turn_started_event() {
     let out = ep.collect_thread_events(&event(
         "t1",
         EventMsg::TurnStarted(codex_core::protocol::TurnStartedEvent {
+            turn_id: "turn-1".to_string(),
             model_context_window: Some(32_000),
             collaboration_mode_kind: ModeKind::Default,
         }),
@@ -310,6 +311,7 @@ fn plan_update_emits_todo_list_started_updated_and_completed() {
     let complete = event(
         "p3",
         EventMsg::TurnComplete(codex_core::protocol::TurnCompleteEvent {
+            turn_id: "turn-1".to_string(),
             last_agent_message: None,
         }),
     );
@@ -677,6 +679,7 @@ fn plan_update_after_complete_starts_new_todo_list_with_new_id() {
     let complete = event(
         "t2",
         EventMsg::TurnComplete(codex_core::protocol::TurnCompleteEvent {
+            turn_id: "turn-1".to_string(),
             last_agent_message: None,
         }),
     );
@@ -829,6 +832,7 @@ fn error_followed_by_task_complete_produces_turn_failed() {
     let complete_event = event(
         "e2",
         EventMsg::TurnComplete(codex_core::protocol::TurnCompleteEvent {
+            turn_id: "turn-1".to_string(),
             last_agent_message: None,
         }),
     );
@@ -1276,6 +1280,7 @@ fn task_complete_produces_turn_completed_with_usage() {
     let complete_event = event(
         "e2",
         EventMsg::TurnComplete(codex_core::protocol::TurnCompleteEvent {
+            turn_id: "turn-1".to_string(),
             last_agent_message: Some("done".to_string()),
         }),
     );