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/tasks/mod.rs

@@ -50,6 +50,7 @@ use codex_protocol::protocol::WarningEvent;
 use codex_protocol::user_input::UserInput;
 
 use codex_features::Feature;
+use codex_protocol::models::ContentItem;
 pub(crate) use compact::CompactTask;
 pub(crate) use ghost_snapshot::GhostSnapshotTask;
 pub(crate) use regular::RegularTask;
@@ -63,10 +64,26 @@ const GRACEFULL_INTERRUPTION_TIMEOUT_MS: u64 = 100;
 
 /// Shared model-visible marker used by both the real interrupt path and
 /// interrupted fork snapshots.
-pub(crate) fn interrupted_turn_history_marker() -> ResponseItem {
-    ContextualUserFragment::into(crate::context::TurnAborted::new(
-        crate::context::TurnAborted::INTERRUPTED_GUIDANCE,
-    ))
+pub(crate) fn interrupted_turn_history_marker(multi_agent_v2_enabled: bool) -> ResponseItem {
+    let guidance = if multi_agent_v2_enabled {
+        crate::context::TurnAborted::INTERRUPTED_DEVELOPER_GUIDANCE
+    } else {
+        crate::context::TurnAborted::INTERRUPTED_GUIDANCE
+    };
+    let marker = crate::context::TurnAborted::new(guidance);
+    if multi_agent_v2_enabled {
+        ResponseItem::Message {
+            id: None,
+            role: "developer".to_string(),
+            content: vec![ContentItem::InputText {
+                text: marker.render(),
+            }],
+            end_turn: None,
+            phase: None,
+        }
+    } else {
+        ContextualUserFragment::into(marker)
+    }
 }
 
 fn emit_turn_network_proxy_metric(
@@ -675,7 +692,7 @@ impl Session {
         if reason == TurnAbortReason::Interrupted {
             self.cleanup_after_interrupt(&task.turn_context).await;
 
-            let marker = interrupted_turn_history_marker();
+            let marker = interrupted_turn_history_marker(self.enabled(Feature::MultiAgentV2));
             self.record_into_history(std::slice::from_ref(&marker), task.turn_context.as_ref())
                 .await;
             self.persist_rollout_items(&[RolloutItem::ResponseItem(marker)])