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/agent/control_tests.rs

@@ -163,6 +163,7 @@ async fn persisted_originator(thread: &CodexThread) -> String {
             RolloutItem::SessionMeta(meta_line) => Some(meta_line.meta.originator.clone()),
             RolloutItem::ResponseItem(_)
             | RolloutItem::InterAgentCommunication(_)
+            | RolloutItem::InterAgentCommunicationMetadata { .. }
             | RolloutItem::EventMsg(_)
             | RolloutItem::Compacted(_)
             | RolloutItem::TurnContext(_) => None,