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- Note that the branch name is out of date because I originally re-added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

@@ -1850,6 +1850,10 @@ pub struct TurnCompleteEvent {
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
 pub struct TurnStartedEvent {
     pub turn_id: String,
+    // Persist for rollout consumers that correlate turns with telemetry traces.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[ts(optional)]
+    pub trace_id: Option<String>,
     /// Unix timestamp (in seconds) when the turn started.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     #[ts(type = "number | null", optional)]