fix(langfuse): capture turn output

[wochinge]

Summary

• Capture the final assistant response on the root opencode.turn span so the turn has both input and output, matching the behavior expected from Langfuse integrations.
• Reuse the same assistant output payload for the turn span and generation span, and centralize turn lookup through getTurnObservation.

Linear

• None

Major Decisions

• Keep output capture on successful generation completion rather than changing turn lifecycle handling, since that is the point where the assistant response text is available.

Review Focus

• Confirm the fallback from parentID to the latest session turn matches OpenCode event ordering.

[chatgpt-codex-connector]

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