fix(tracing): nest subagent turns under their spawn_agent call

[pushkar]

Subagent turns currently nest under the parent turn root, so in Langfuse they show up as siblings of the spawn_agent call that started them instead of under it. This parents each subagent rollout on its matching spawn_agent tool observation.

Changes

• parseSession records the spawn call id for each subagent thread (subagentSpawnCallIds), from spawn_agent tool output, collab_agent_spawn_end, and sub_agent_activity.
• Also recognizes the item_completed / SubAgentActivity layout that Codex 0.157.x writes for subagent starts.
• emitTurn parents the child rollout on the matching spawn_agent observation, falling back to the turn root when there's no match. So the old behavior is the fallback, not a regression.

Testing

• pnpm lint and pnpm test pass (121 tests).
• New parse test for the item_completed layout. Existing parse tests now assert the thread to call-id mapping.
• Trace tests assert that the subagent turn's parent is the spawn_agent span, in both standalone and attached (external parent) mode.

Before	After

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}