feat(core): persist trace_id for turns in RolloutItem::TurnContext

[owenlin0]

This PR adds a durable trace linkage for each turn by storing the active trace ID on the rollout TurnContext record stored in session rollout files.

Before this change, we propagated trace context at runtime but didn’t persist a stable per-turn trace key in rollout history. That made after-the-fact debugging harder (for example, mapping a historical turn to the corresponding trace in datadog). This sets us up for much easier debugging in the future.

What changed

• Added an optional trace_id to TurnContextItem (rollout schema).
• Added a small OTEL helper to read the current span trace ID.
• Captured trace_id when creating TurnContext and included it in to_turn_context_item().
• Updated tests and fixtures that construct TurnContextItem so older/no-trace cases still work.

Why this approach

TurnContext is already the canonical durable per-turn metadata in rollout. This keeps ownership clean: trace linkage lives with other persisted turn metadata.

[owenlin0]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Chef's kiss.

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