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/app-server-protocol/src/protocol/thread_history.rs

@@ -386,6 +386,7 @@ impl ThreadHistoryBuilder {
             RolloutItem::Compacted(payload) => self.handle_compacted(payload),
             RolloutItem::ResponseItem(item) => self.handle_response_item(item),
             RolloutItem::InterAgentCommunication(_)
+            | RolloutItem::InterAgentCommunicationMetadata { .. }
             | RolloutItem::TurnContext(_)
             | RolloutItem::SessionMeta(_) => {}
         }