test protocol: lock inter-agent commentary phase (#20046)

## Summary
- add a regression test for
`InterAgentCommunication::to_response_input_item`
- assert replayed inter-agent messages keep `phase:
Some(MessagePhase::Commentary)`

## Test plan
- `cargo test -p codex-protocol`
- `just argument-comment-lint`

codex-rs/protocol/src/protocol.rs

@@ -4019,6 +4019,28 @@ mod tests {
         );
     }
 
+    #[test]
+    fn inter_agent_communication_response_input_item_preserves_commentary_phase() {
+        let communication = InterAgentCommunication {
+            author: AgentPath::root(),
+            recipient: AgentPath::root().join("reviewer").expect("recipient path"),
+            other_recipients: vec![AgentPath::root().join("worker").expect("recipient path")],
+            content: "review the diff".to_string(),
+            trigger_turn: true,
+        };
+
+        assert_eq!(
+            communication.to_response_input_item(),
+            ResponseInputItem::Message {
+                role: "assistant".to_string(),
+                content: vec![ContentItem::OutputText {
+                    text: serde_json::to_string(&communication).expect("serialize communication"),
+                }],
+                phase: Some(MessagePhase::Commentary),
+            }
+        );
+    }
+
     #[test]
     fn session_source_from_startup_arg_normalizes_custom_values() {
         assert_eq!(