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- Note that the branch name is out of date because I originally re-added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/app-server/src/request_processors/thread_processor_tests.rs

@@ -1155,6 +1155,7 @@ mod thread_processor_behavior_tests {
                 "turn-1",
                 &EventMsg::TurnStarted(codex_protocol::protocol::TurnStartedEvent {
                     turn_id: "turn-1".to_string(),
+                    trace_id: None,
                     started_at: None,
                     model_context_window: None,
                     collaboration_mode_kind: Default::default(),