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/tests.rs

@@ -1766,6 +1766,29 @@ async fn record_inter_agent_communication_sets_turn_id_in_rollout_and_resume() {
     else {
         panic!("expected resumed rollout history");
     };
+    let persisted_items = resumed
+        .history
+        .iter()
+        .filter(|item| {
+            matches!(
+                item,
+                RolloutItem::ResponseItem(_)
+                    | RolloutItem::InterAgentCommunication(_)
+                    | RolloutItem::InterAgentCommunicationMetadata { .. }
+            )
+        })
+        .cloned()
+        .collect::<Vec<_>>();
+    let expected_persisted_items = vec![
+        RolloutItem::InterAgentCommunicationMetadata {
+            trigger_turn: false,
+        },
+        RolloutItem::ResponseItem(expected_item.clone()),
+    ];
+    assert_eq!(
+        serde_json::to_value(persisted_items).unwrap(),
+        serde_json::to_value(expected_persisted_items).unwrap()
+    );
 
     let (resumed_session, _resumed_turn_context) = make_session_and_context().await;
     resumed_session
@@ -1818,14 +1841,11 @@ async fn record_inter_agent_communication_preserves_item_id_in_rollout_and_resum
     else {
         panic!("expected resumed rollout history");
     };
-    let persisted_communication = resumed.history.iter().find_map(|item| match item {
-        RolloutItem::InterAgentCommunication(communication) => Some(communication),
+    let persisted_item_id = resumed.history.iter().find_map(|item| match item {
+        RolloutItem::ResponseItem(item @ ResponseItem::AgentMessage { .. }) => item.id(),
         _ => None,
     });
-    assert_eq!(
-        persisted_communication.and_then(|communication| communication.id.as_deref()),
-        Some(live_item_id.as_str())
-    );
+    assert_eq!(persisted_item_id, Some(live_item_id.as_str()));
 
     let (resumed_session, _resumed_turn_context, _rx) =
         make_session_and_context_with_auth_and_config_and_rx(
@@ -2727,6 +2747,7 @@ async fn start_new_context_window_assigns_and_persists_item_ids() {
         RolloutItem::SessionMeta(_)
         | RolloutItem::ResponseItem(_)
         | RolloutItem::InterAgentCommunication(_)
+        | RolloutItem::InterAgentCommunicationMetadata { .. }
         | RolloutItem::TurnContext(_)
         | RolloutItem::EventMsg(_) => None,
     });
@@ -2783,6 +2804,7 @@ async fn record_initial_history_assigns_and_persists_id_for_forked_response_item
         RolloutItem::ResponseItem(response_item) => response_item.id(),
         RolloutItem::SessionMeta(_)
         | RolloutItem::InterAgentCommunication(_)
+        | RolloutItem::InterAgentCommunicationMetadata { .. }
         | RolloutItem::Compacted(_)
         | RolloutItem::TurnContext(_)
         | RolloutItem::EventMsg(_) => None,