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/protocol/src/models.rs

@@ -719,6 +719,7 @@ pub enum ContentItem {
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, JsonSchema, TS)]
 #[serde(tag = "type", rename_all = "snake_case")]
 pub enum AgentMessageInputContent {
+    InputText { text: String },
     EncryptedContent { encrypted_content: String },
 }
 

codex-rs/protocol/src/protocol.rs

@@ -528,7 +528,7 @@ pub enum Op {
         thread_settings: ThreadSettingsOverrides,
     },
 
-    /// Inter-agent communication that should be recorded as assistant history
+    /// Inter-agent communication that should be recorded as agent-message history
     /// while still using the normal thread submission lifecycle.
     InterAgentCommunication {
         communication: InterAgentCommunication,
@@ -714,15 +714,18 @@ impl InterAgentCommunication {
     }
 
     pub fn to_model_input_item(&self) -> ResponseItem {
-        match &self.encrypted_content {
-            Some(encrypted_content) => ResponseItem::AgentMessage {
-                author: self.author.to_string(),
-                recipient: self.recipient.to_string(),
-                content: vec![AgentMessageInputContent::EncryptedContent {
-                    encrypted_content: encrypted_content.clone(),
-                }],
+        let content = match &self.encrypted_content {
+            Some(encrypted_content) => AgentMessageInputContent::EncryptedContent {
+                encrypted_content: encrypted_content.clone(),
             },
-            None => self.to_response_input_item().into(),
+            None => AgentMessageInputContent::InputText {
+                text: self.content.clone(),
+            },
+        };
+        ResponseItem::AgentMessage {
+            author: self.author.to_string(),
+            recipient: self.recipient.to_string(),
+            content: vec![content],
         }
     }
 
@@ -2800,6 +2803,7 @@ fn multi_agent_version_from_items(
             RolloutItem::TurnContext(turn_context) => turn_context.multi_agent_version,
             RolloutItem::SessionMeta(_)
             | RolloutItem::ResponseItem(_)
+            | RolloutItem::InterAgentCommunication(_)
             | RolloutItem::Compacted(_)
             | RolloutItem::EventMsg(_) => None,
         })
@@ -2895,6 +2899,8 @@ pub struct SessionMetaLine {
 pub enum RolloutItem {
     SessionMeta(SessionMetaLine),
     ResponseItem(ResponseItem),
+    /// Durable delivery metadata reconstructed as a model-visible `agent_message`.
+    InterAgentCommunication(InterAgentCommunication),
     Compacted(CompactedItem),
     TurnContext(TurnContextItem),
     EventMsg(EventMsg),