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/ext/web-search/src/history.rs

@@ -1,6 +1,7 @@
 use codex_api::SearchInput;
 use codex_core::parse_turn_item;
 use codex_protocol::items::TurnItem;
+use codex_protocol::models::AgentMessageInputContent;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::ResponseItem;
 use codex_tools::retain_tail_from_last_n_user_messages;
@@ -30,6 +31,28 @@ fn push_visible_message(messages: &mut Vec<ResponseItem>, item: &ResponseItem) {
         ResponseItem::Message { role, .. } if role == ASSISTANT_ROLE => {
             messages.push(item.clone());
         }
+        ResponseItem::AgentMessage {
+            author, content, ..
+        } => {
+            let text = content
+                .iter()
+                .filter_map(|content| match content {
+                    AgentMessageInputContent::InputText { text } => Some(text.as_str()),
+                    AgentMessageInputContent::EncryptedContent { .. } => None,
+                })
+                .collect::<Vec<_>>()
+                .join("\n");
+            if !text.trim().is_empty() {
+                messages.push(ResponseItem::Message {
+                    id: None,
+                    role: ASSISTANT_ROLE.to_string(),
+                    content: vec![ContentItem::OutputText {
+                        text: format!("Agent message from {author}:\n{text}"),
+                    }],
+                    phase: None,
+                });
+            }
+        }
         ResponseItem::Message {
             id,
             role,