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/rollout/src/list.rs

@@ -1117,11 +1117,10 @@ async fn read_head_summary(path: &Path, head_limit: usize) -> io::Result<HeadTai
                     summary.saw_session_meta = true;
                 }
             }
-            RolloutItem::ResponseItem(_) => {
-                summary.created_at = summary
+            RolloutItem::ResponseItem(_) | RolloutItem::InterAgentCommunication(_) => {
+                summary
                     .created_at
-                    .clone()
-                    .or_else(|| Some(rollout_line.timestamp.clone()));
+                    .get_or_insert_with(|| rollout_line.timestamp.clone());
             }
             RolloutItem::TurnContext(_) => {
                 // Not included in `head`; skip.
@@ -1180,6 +1179,11 @@ pub async fn read_head_for_summary(path: &Path) -> io::Result<Vec<serde_json::Va
                         head.push(value);
                     }
                 }
+                RolloutItem::InterAgentCommunication(communication) => {
+                    if let Ok(value) = serde_json::to_value(communication.to_model_input_item()) {
+                        head.push(value);
+                    }
+                }
                 RolloutItem::Compacted(_)
                 | RolloutItem::TurnContext(_)
                 | RolloutItem::EventMsg(_) => {}