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/client_common.rs

@@ -5,7 +5,6 @@ use codex_protocol::models::BaseInstructions;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::FunctionCallOutputContentItem;
 use codex_protocol::models::ResponseItem;
-use codex_protocol::protocol::InterAgentCommunication;
 use codex_tools::ToolSpec;
 use futures::Stream;
 use serde_json::Value;
@@ -55,30 +54,11 @@ impl Default for Prompt {
 }
 
 impl Prompt {
-    pub(crate) fn get_formatted_input(&self) -> Vec<ResponseItem> {
-        self.input
-            .iter()
-            .cloned()
-            .map(|item| {
-                let ResponseItem::Message { role, content, .. } = &item else {
-                    return item;
-                };
-                if role != "assistant" {
-                    return item;
-                }
-                InterAgentCommunication::from_message_content(content)
-                    .filter(|communication| communication.encrypted_content.is_some())
-                    .map(|communication| communication.to_model_input_item())
-                    .unwrap_or(item)
-            })
-            .collect()
-    }
-
     pub(crate) fn get_formatted_input_for_request(
         &self,
         use_responses_lite: bool,
     ) -> Vec<ResponseItem> {
-        let mut input = self.get_formatted_input();
+        let mut input = self.input.clone();
         if use_responses_lite {
             strip_image_details(&mut input);
         }