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- Note that the branch name is out of date because I originally re-added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/src/tasks/regular.rs

@@ -51,6 +51,7 @@ impl SessionTask for RegularTask {
         // not wait on startup prewarm resolution.
         let event = EventMsg::TurnStarted(TurnStartedEvent {
             turn_id: ctx.sub_id.clone(),
+            trace_id: ctx.trace_id.clone(),
             started_at: ctx.turn_timing_state.started_at_unix_secs().await,
             model_context_window: ctx.model_context_window(),
             collaboration_mode_kind: ctx.collaboration_mode.mode,

codex-rs/core/src/tasks/user_shell.rs

@@ -114,6 +114,7 @@ pub(crate) async fn execute_user_shell_command(
         // freshly reinjected context before the summary/replacement history is applied.
         let event = EventMsg::TurnStarted(TurnStartedEvent {
             turn_id: turn_context.sub_id.clone(),
+            trace_id: turn_context.trace_id.clone(),
             started_at: turn_context.turn_timing_state.started_at_unix_secs().await,
             model_context_window: turn_context.model_context_window(),
             collaboration_mode_kind: turn_context.collaboration_mode.mode,