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/core/src/session/mod.rs

@@ -1540,6 +1540,7 @@ impl Session {
     ) -> Option<PreviousTurnSettings> {
         let rollout_reconstruction::RolloutReconstruction {
             mut history,
+            retained_context,
             guardian_history,
             previous_turn_settings,
             reference_context_item,
@@ -1586,6 +1587,7 @@ impl Session {
             state
                 .history
                 .restore_guardian_history(guardian_history.as_ref());
+            state.history.restore_retained_context(&retained_context);
             if let Some(world_state) = world_state_baseline {
                 state.history.set_world_state_baseline(world_state);
             }
@@ -3760,6 +3762,7 @@ impl Session {
         let mut compacted_item = CompactedItem {
             message: metadata.message,
             replacement_history: Some(items.clone()),
+            retained_context: None,
             guardian_history: None,
             mcp_resource_origins: self.services.mcp_runtime.resource_origin_checkpoint(),
             window_number: Some(metadata.window_number),
@@ -3785,6 +3788,7 @@ impl Session {
                 HistoryReplacement::Compaction,
             );
             compacted_item.guardian_history = state.history.guardian_history_checkpoint();
+            compacted_item.retained_context = Some(state.history.retained_context().clone());
             if let Some(world_state) = world_state_baseline {
                 let snapshot = world_state.snapshot();
                 world_state_item = Some(WorldStateItem::full(snapshot.clone().into_object()));