Persist agent messages as response items (#29829)

## Why

Inter-agent messages are recorded in live history as
`ResponseItem::AgentMessage`, but rollouts stored
`InterAgentCommunication` and rebuilt the response item during resume.
This made the rollout differ from the actual Responses history.

## What changed

- store the prepared `agent_message` response item directly
- keep `trigger_turn` in a small local metadata record for fork
truncation
- keep reading older `inter_agent_communication` rollout items

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

@@ -264,6 +264,7 @@ impl Session {
                         active_segment.get_or_insert_with(ActiveReplaySegment::default);
                     active_segment.counts_as_user_turn = true;
                 }
+                RolloutItem::InterAgentCommunicationMetadata { .. } => {}
                 RolloutItem::EventMsg(_) | RolloutItem::SessionMeta(_) => {}
             }
 
@@ -320,6 +321,7 @@ impl Session {
                         turn_context.model_info.truncation_policy.into(),
                     );
                 }
+                RolloutItem::InterAgentCommunicationMetadata { .. } => {}
                 RolloutItem::Compacted(compacted) => {
                     if let Some(replacement_history) = &compacted.replacement_history {
                         // This should actually never happen, because the reverse loop above (to build rollout_suffix)