Make MultiAgentV2 interruption markers assistant-authored (#19124)

## Why

`MultiAgentV2` follow-up messages are delivered to agents as
assistant-authored `InterAgentCommunication` envelopes. When
`followup_task` used `interrupt: true`, the interrupted-turn guidance
was still persisted as a contextual user message, so model-visible
history made a system-generated interruption boundary look
user-authored.

This keeps interruption guidance consistent with the rest of the v2
inter-agent message stream while preserving the legacy marker shape for
non-v2 sessions.

## What changed

- Make `interrupted_turn_history_marker` feature-aware.
- Record the interrupted-turn marker as an assistant `OutputText`
message when `Feature::MultiAgentV2` is enabled.
- Keep the existing user contextual fragment for non-v2 sessions.
- Apply the same feature-aware marker to interrupted fork snapshots.
- Add coverage for the live `followup_task` interrupt path and the
helper-level v2 marker shape.

## Testing

- `cargo test -p codex-core
multi_agent_v2_followup_task_interrupts_busy_child_without_losing_message
-- --nocapture`
- `cargo test -p codex-core
multi_agent_v2_interrupted_marker_uses_assistant_output_message --
--nocapture`
- `cargo test -p codex-core interrupted_fork_snapshot -- --nocapture`

codex-rs/core/src/tools/handlers/multi_agents_tests.rs

@@ -3,6 +3,7 @@ use crate::CodexThread;
 use crate::ThreadManager;
 use crate::config::AgentRoleConfig;
 use crate::config::DEFAULT_AGENT_MAX_DEPTH;
+use crate::context::TurnAborted;
 use crate::function_tool::FunctionCallError;
 use crate::session::tests::make_session_and_context;
 use crate::session_prefix::format_subagent_notification_message;
@@ -1566,6 +1567,52 @@ async fn multi_agent_v2_followup_task_interrupts_busy_child_without_losing_messa
     }));
 
     wait_for_turn_aborted(&thread, &interrupted_turn_id, TurnAbortReason::Interrupted).await;
+    let history_items = thread
+        .codex
+        .session
+        .clone_history()
+        .await
+        .raw_items()
+        .to_vec();
+    assert!(
+        history_items.iter().any(|item| matches!(
+            item,
+            ResponseItem::Message { role, content, .. }
+                if role == "developer"
+                    && content.iter().any(|content_item| matches!(
+                        content_item,
+                        ContentItem::InputText { text }
+                            if text.contains(TurnAborted::INTERRUPTED_DEVELOPER_GUIDANCE)
+                    ))
+        )),
+        "v2 interrupted-turn marker should be recorded as a developer input message"
+    );
+    assert!(
+        !history_items.iter().any(|item| matches!(
+            item,
+            ResponseItem::Message { role, content, .. }
+                if role == "user"
+                    && content.iter().any(|content_item| matches!(
+                        content_item,
+                        ContentItem::InputText { text } | ContentItem::OutputText { text }
+                            if text.contains(TurnAborted::INTERRUPTED_GUIDANCE)
+                    ))
+        )),
+        "v2 interrupted-turn marker should not be recorded as a user message"
+    );
+    assert!(
+        !history_items.iter().any(|item| matches!(
+            item,
+            ResponseItem::Message { role, content, .. }
+                if role == "assistant"
+                    && content.iter().any(|content_item| matches!(
+                        content_item,
+                        ContentItem::InputText { text } | ContentItem::OutputText { text }
+                            if text.contains(TurnAborted::INTERRUPTED_DEVELOPER_GUIDANCE)
+                    ))
+        )),
+        "v2 interrupted-turn marker should not be recorded as an assistant message"
+    );
     wait_for_redirected_envelope_in_history(
         &thread,
         &InterAgentCommunication::new(