Preserve context baselines across nested agent forks (#41424)

## What changed

- Treat a surviving full world-state snapshot as a context baseline when a fork removes the associated user message.
- Restore the previous turn settings and reference context from that baseline, without treating the segment as a user turn for rollback.
- Ignore partial snapshots and full snapshots superseded by compaction.

## Testing

- Cover resumed rollout reconstruction with removed task messages, partial snapshots, and compaction.
- Verify nested agents inherit developer instructions and environment context exactly once across history modes and compacted histories.

GitOrigin-RevId: 9f9f6992d7ea8cff9ac82a7fcdf0c8fb764db81c

codex-rs/core/src/session/rollout_reconstruction.rs

@@ -78,6 +78,16 @@ fn finalize_active_segment<'a>(
         return;
     }
 
+    // Full world-state snapshots are persisted after installing initial context. They still
+    // establish a baseline when a child fork removes the parent turn's agent message. Do not
+    // count these context-only segments as user turns for rollback, or use a snapshot from
+    // before the segment's latest compaction.
+    let has_context_baseline = active_segment.counts_as_user_turn
+        || active_segment
+            .world_state_replay
+            .iter()
+            .take_while(|item| !matches!(item, RolloutItem::Compacted(_)))
+            .any(|item| matches!(item, RolloutItem::WorldState(state) if state.full));
     world_state_replay.extend(active_segment.world_state_replay);
 
     // A surviving replacement-history checkpoint is a complete history base. Once we
@@ -92,15 +102,15 @@ fn finalize_active_segment<'a>(
         *window = active_segment.window;
     }
 
-    // `previous_turn_settings` come from the newest surviving user turn that established them.
-    if previous_turn_settings.is_none() && active_segment.counts_as_user_turn {
+    // Restore settings from the newest surviving context baseline.
+    if previous_turn_settings.is_none() && has_context_baseline {
         *previous_turn_settings = active_segment.previous_turn_settings;
     }
 
-    // `reference_context_item` comes from the newest surviving user turn baseline, or
+    // `reference_context_item` comes from the newest surviving context baseline, or
     // from a surviving compaction that explicitly cleared that baseline.
     if matches!(reference_context_item, TurnReferenceContextItem::NeverSet)
-        && (active_segment.counts_as_user_turn
+        && (has_context_baseline
             || matches!(
                 active_segment.reference_context_item,
                 TurnReferenceContextItem::Cleared