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/rollout_reconstruction.rs

@@ -220,6 +220,11 @@ impl Session {
                         active_segment.get_or_insert_with(ActiveReplaySegment::default);
                     active_segment.counts_as_user_turn |= is_user_turn_boundary(response_item);
                 }
+                RolloutItem::InterAgentCommunication(_) => {
+                    let active_segment =
+                        active_segment.get_or_insert_with(ActiveReplaySegment::default);
+                    active_segment.counts_as_user_turn = true;
+                }
                 RolloutItem::EventMsg(_) | RolloutItem::SessionMeta(_) => {}
             }
 
@@ -269,6 +274,13 @@ impl Session {
                         turn_context.truncation_policy,
                     );
                 }
+                RolloutItem::InterAgentCommunication(communication) => {
+                    let response_item = communication.to_model_input_item();
+                    history.record_items(
+                        std::iter::once(&response_item),
+                        turn_context.truncation_policy,
+                    );
+                }
                 RolloutItem::Compacted(compacted) => {
                     if let Some(replacement_history) = &compacted.replacement_history {
                         // This should actually never happen, because the reverse loop above (to build rollout_suffix)