Add trace_id to TurnStartedEvent (#23980)

## Why
[Recent PR](https://github.com/openai/codex/pull/22709) removed
`trace_id` from `TurnContextItem`.

## What changed
- Add to `TurnStartedEvent` so rollout consumers can correlate turns
with telemetry traces.
- Note that the branch name is out of date because I originally re-added
to `TurnContextItem`, but we decided to move it to `TurnStartedEvent`.

## Verification
- `cargo test -p codex-protocol`
- `cargo test -p codex-core --lib
regular_turn_emits_turn_started_without_waiting_for_startup_prewarm`
- `cargo test -p codex-core --test all
emits_warning_when_resumed_model_differs`
- `cargo test -p codex-rollout`
- `cargo test -p codex-state`

codex-rs/core/tests/suite/resume_warning.rs

@@ -50,6 +50,7 @@ fn resume_history(
         history: vec![
             RolloutItem::EventMsg(EventMsg::TurnStarted(TurnStartedEvent {
                 turn_id: turn_id.clone(),
+                trace_id: None,
                 started_at: None,
                 model_context_window: None,
                 collaboration_mode_kind: ModeKind::Default,