Preserve verified answers across history compaction (#42293)

## Why

Compaction replaces the model history window, but host-verified user answers may
still be needed for the rest of the owning thread.

## What changed

- Add bounded, model-invisible retained context for verified
  `request_user_input` answers.
- Persist retained context in rollout events and compaction checkpoints, and
  restore it when a thread resumes.
- Remove answers when their source turns are rolled back, and prevent
  parent-local answers from being inherited by child forks.

## Testing

Add lifecycle coverage for deduplication and size limits, compaction and resume,
rollback, rollout migration, and child forks.

GitOrigin-RevId: c32900d3e8120a9899b71f0576683acd67a4b898

codex-rs/rollout/src/model_context.rs

@@ -160,6 +160,7 @@ impl ModelContextScan {
             | RolloutItem::SessionMeta(_)
             | RolloutItem::InterAgentCommunicationMetadata { .. }
             | RolloutItem::RealtimeItem(_)
+            | RolloutItem::RetainedContext(_)
             | RolloutItem::SecurityRiskScore(_)
             | RolloutItem::TokenUsageRecord(_) => {}
         }