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/external-agent-migration/src/sessions/append.rs

@@ -244,6 +244,7 @@ fn source_model_items(items: &[RolloutItem]) -> Option<Vec<SourceModelItem<'_>>>
             | RolloutItem::InterAgentCommunication(_)
             | RolloutItem::Compacted(_)
             | RolloutItem::TurnContext(_)
+            | RolloutItem::RetainedContext(_)
             | RolloutItem::SecurityRiskScore(_)
             | RolloutItem::WorldState(_) => return None,
             RolloutItem::EventMsg(_) => {}
@@ -260,6 +261,7 @@ fn history_model_items(items: &[RolloutItem]) -> Option<Vec<&ResponseItem>> {
             | RolloutItem::InterAgentCommunicationMetadata { .. }
             | RolloutItem::TokenUsageRecord(_)
             | RolloutItem::RealtimeItem(_)
+            | RolloutItem::RetainedContext(_)
             | RolloutItem::SecurityRiskScore(_) => {}
             RolloutItem::ResponseItem(response_item) => model_items.push(&response_item.item),
             RolloutItem::EventMsg(