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- Note that the branch name is out of date because I originally re-added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

@@ -62,6 +62,7 @@ pub(crate) async fn run_remote_compact_task(
 ) -> CodexResult<()> {
     let start_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,