Support plaintext agent messages (#27830)

## Why

Multi-agent v2 `send_message` deliveries already reach the receiving
model as typed `agent_message` items with encrypted content.
Child-completion notifications are generated by Codex itself, so their
content is plaintext and previously fell back to a serialized JSON
envelope inside an assistant message.

With plaintext `input_text` supported for `agent_message`, both delivery
paths can use the same model-visible type while preserving explicit
author and recipient metadata.

## What changed

- add plaintext `input_text` support to `AgentMessageInputContent` and
regenerate the affected app-server schemas
- preserve `InterAgentCommunication` as structured mailbox input instead
of converting it to assistant text
- record delivered communications as typed `agent_message` history items
- persist a dedicated rollout item so local delivery metadata such as
`trigger_turn` remains available without leaking into the Responses
request
- reconstruct typed agent messages on resume and preserve fork-turn
truncation behavior
- remove request-time assistant-content parsing
- preserve plaintext and encrypted inter-agent deliveries in stage-one
memory inputs
- normalize and link plaintext and encrypted agent messages in rollout
traces without treating inbound messages as child results
- cover the real MultiAgent V2 child-completion path end to end with
deterministic mailbox synchronization

## Verification

- `just test -p codex-core
plaintext_multi_agent_v2_completion_sends_agent_message`
- `just test -p codex-core input_queue_drains_mailbox_in_delivery_order
record_initial_history_reconstructs_typed_inter_agent_message
fork_turn_positions_use_inter_agent_delivery_metadata`
- `just test -p codex-memories-write
serializes_inter_agent_communications_for_memory`
- `just test -p codex-rollout-trace
agent_messages_preserve_routing_and_content
sub_agent_started_activity_creates_spawn_edge`
- `just test -p codex-rollout-trace
agent_result_edge_falls_back_to_child_thread_without_result_message`
- `just test -p codex-protocol -p codex-rollout -p
codex-app-server-protocol`

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

@@ -2659,6 +2659,26 @@ impl Session {
         self.send_raw_response_items(turn_context, items).await;
     }
 
+    pub(crate) async fn record_inter_agent_communication(
+        &self,
+        turn_context: &TurnContext,
+        communication: InterAgentCommunication,
+    ) {
+        let response_item = communication.to_model_input_item();
+        let items = self.prepare_conversation_items_for_history(
+            turn_context,
+            std::slice::from_ref(&response_item),
+        );
+        let items = items.as_ref();
+        {
+            let mut state = self.state.lock().await;
+            state.record_items(items.iter(), turn_context.truncation_policy);
+        }
+        self.persist_rollout_items(&[RolloutItem::InterAgentCommunication(communication)])
+            .await;
+        self.send_raw_response_items(turn_context, items).await;
+    }
+
     async fn maybe_warn_on_server_model_mismatch(
         self: &Arc<Self>,
         turn_context: &Arc<TurnContext>,