[codex-analytics] emit internally started turn events

[marksteinbrick-oai]

Why

Currently, the analytics reducer omits codex_turn_event for internally started subagent turns

• It uses TurnState.connection_id to select app-server client and runtime metadata
• turn/start sets this field for client-started turns, while internal subagent turns bypass that path
• Spawned child threads inherit the correct connection, but turn emission does not use thread state

What Changed

• Keeps explicit TurnState.connection_id authoritative for client-started turns
• Falls back to the matching thread’s inherited connection when the turn connection is absent
• Preserves completeness gates, event schema, and post-emission state removal
• Extends subagent lifecycle test coverage

Verification

• just test -p codex-analytics (71 tests passed)
• just fix -p codex-analytics
• just fmt