[codex] Omit fork turns from thread started notifications

[euroelessar]

Why

thread/fork responses intentionally include copied history so the caller can render the fork immediately, but thread/started is a lifecycle notification. The v2 Thread contract says notifications should return turns: [], and the fork path was reusing the response thread directly, causing copied turns to be emitted through thread/started as well.

What Changed

• Route app-server thread/started notification construction through a helper that clears thread.turns before sending.
• Keep thread/fork responses unchanged so callers still receive copied history.
• Add persistent and ephemeral fork coverage that asserts thread/started emits an empty turns array while the response retains fork history.

Testing

• just fmt
• cargo test -p codex-app-server