Support new `sub_agent_activity` events when resolving subagent traces

[yunhungo]

Summary

Subagent turns can be omitted from exported Langfuse traces when Codex rollouts use the newer sub_agent_activity lifecycle event.

The current parser only extracts child thread IDs from:

{
  "type": "collab_agent_spawn_end",
  "new_thread_id": "thread-child"
}

Newer rollouts may represent the same lifecycle transition as:

{
  "type": "sub_agent_activity",
  "kind": "started",
  "agent_thread_id": "thread-child"
}

Because plugins/tracing/src/parse.ts does not recognize the latter shape, turn.subagentThreadIds remains empty and the existing child rollout is not nested as a Codex Subagent Turn.

Expected behavior

Support both formats and deduplicate child thread IDs:

if (et === "collab_agent_spawn_end" && typeof p.new_thread_id === "string") {
  turn.subagentThreadIds.push(p.new_thread_id);
}

if (
  et === "sub_agent_activity" &&
  p.kind === "started" &&
  typeof p.agent_thread_id === "string"
) {
  turn.subagentThreadIds.push(p.agent_thread_id);
}

Suggested test coverage

Add a rollout fixture using sub_agent_activity/agent_thread_id and assert that:

• the child thread ID is captured exactly once;
• its rollout is resolved;
• Codex Subagent Turn is nested under the parent turn;
• the child generations are emitted as LLM Subagent.

This differs from #12: the parent turn export can succeed while only subagent resolution is missing.