[codex] drain mailbox only at request boundaries (#17749)

This changes multi-agent v2 mailbox handling so incoming inter-agent
messages no longer preempt an in-flight sampling stream at reasoning or
commentary output-item boundaries.

codex-rs/core/tests/suite/pending_input.rs

@@ -313,7 +313,7 @@ async fn injected_user_input_triggers_follow_up_request_with_deltas() {
 }
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn queued_inter_agent_mail_triggers_follow_up_after_reasoning_item() {
+async fn queued_inter_agent_mail_waits_for_request_boundary_after_reasoning_item() {
     let (gate_reasoning_done_tx, gate_reasoning_done_rx) = oneshot::channel();
 
     let first_chunks = vec![
@@ -323,14 +323,18 @@ async fn queued_inter_agent_mail_triggers_follow_up_after_reasoning_item() {
             gate_reasoning_done_rx,
             vec![
                 ev_reasoning_item("reason-1", &["thinking"], &[]),
-                ev_function_call(
-                    "call-stale",
-                    "shell",
-                    r#"{"command":"echo stale tool call"}"#,
-                ),
-                ev_message_item_added("msg-stale", ""),
-                ev_output_text_delta("stale final"),
-                ev_message_item_done("msg-stale", "stale final"),
+                ev_message_item_added("msg-preserved", ""),
+                ev_output_text_delta("preserved commentary"),
+                json!({
+                    "type": "response.output_item.done",
+                    "item": {
+                        "type": "message",
+                        "role": "assistant",
+                        "id": "msg-preserved",
+                        "content": [{"type": "output_text", "text": "preserved commentary"}],
+                        "phase": "commentary",
+                    }
+                }),
                 ev_completed("resp-1"),
             ],
         ),
@@ -358,7 +362,7 @@ async fn queued_inter_agent_mail_triggers_follow_up_after_reasoning_item() {
 }
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn queued_inter_agent_mail_triggers_follow_up_after_commentary_message_item() {
+async fn queued_inter_agent_mail_waits_for_request_boundary_after_commentary_message_item() {
     let (gate_message_done_tx, gate_message_done_rx) = oneshot::channel();
 
     let first_chunks = vec![
@@ -367,25 +371,18 @@ async fn queued_inter_agent_mail_triggers_follow_up_after_commentary_message_ite
         gated_chunk(
             gate_message_done_rx,
             vec![
-                ev_output_text_delta("first answer"),
+                ev_output_text_delta("first commentary"),
                 json!({
                     "type": "response.output_item.done",
                     "item": {
                         "type": "message",
                         "role": "assistant",
                         "id": "msg-1",
-                        "content": [{"type": "output_text", "text": "first answer"}],
+                        "content": [{"type": "output_text", "text": "first commentary"}],
                         "phase": "commentary",
                     }
                 }),
-                ev_function_call(
-                    "call-stale",
-                    "shell",
-                    r#"{"command":"echo stale tool call"}"#,
-                ),
-                ev_message_item_added("msg-stale", ""),
-                ev_output_text_delta("stale final"),
-                ev_message_item_done("msg-stale", "stale final"),
+                ev_function_call("call-preserved", "test_tool", "{}"),
                 ev_completed("resp-1"),
             ],
         ),
@@ -411,7 +408,7 @@ async fn queued_inter_agent_mail_triggers_follow_up_after_commentary_message_ite
 
     let _ = gate_message_done_tx.send(());
 
-    wait_for_agent_message(&codex, "first answer").await;
+    wait_for_agent_message(&codex, "first commentary").await;
 
     wait_for_turn_complete(&codex).await;
 

codex-rs/core/tests/suite/snapshots/all__suite__pending_input__pending_input_queued_mail_after_commentary.snap

@@ -13,5 +13,7 @@ Scenario: /responses POST bodies (input only, redacted like other suite snapshot
 00:message/developer:<PERMISSIONS_INSTRUCTIONS>
 01:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
 02:message/user:first prompt
-03:message/assistant:first answer
-04:message/assistant:{"author":"/root/worker","recipient":"/root","other_recipients":[],"content":"queued child update","trigger_turn":false}
+03:message/assistant:first commentary
+04:function_call/test_tool
+05:function_call_output:unsupported call: test_tool
+06:message/assistant:{"author":"/root/worker","recipient":"/root","other_recipients":[],"content":"queued child update","trigger_turn":false}

codex-rs/core/tests/suite/snapshots/all__suite__pending_input__pending_input_queued_mail_after_reasoning.snap

@@ -14,4 +14,5 @@ Scenario: /responses POST bodies (input only, redacted like other suite snapshot
 01:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
 02:message/user:first prompt
 03:reasoning:summary=thinking:encrypted=true
-04:message/assistant:{"author":"/root/worker","recipient":"/root","other_recipients":[],"content":"queued child update","trigger_turn":false}
+04:message/assistant:preserved commentary
+05:message/assistant:{"author":"/root/worker","recipient":"/root","other_recipients":[],"content":"queued child update","trigger_turn":false}