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/spawn.rs

@@ -58,7 +58,8 @@ fn keep_forked_rollout_item(item: &RolloutItem, preserve_reference_context_item:
             | ResponseItem::ContextCompaction { .. }
             | ResponseItem::Other,
         ) => false,
-        RolloutItem::InterAgentCommunication(_) => false,
+        RolloutItem::InterAgentCommunication(_)
+        | RolloutItem::InterAgentCommunicationMetadata { .. } => false,
         // Full-history forks preserve the cached prompt prefix and can keep diffing
         // from the parent's durable baseline. Truncated forks drop part of that prompt,
         // so they must rebuild context on their first child turn.

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,