Revert "[codex] drain mailbox only at request boundaries" (#18325)

## Summary
- Reverts PR #17749 so queued inter-agent mail can again preempt after
reasoning/commentary output item boundaries.
- Applies the revert to the current `codex/turn.rs` module layout and
restores the prior pending-input test expectations/snapshots.

## Testing
- `just fmt`
- `cargo test -p codex-core --test all pending_input`
- `cargo test -p codex-core` failed in unrelated
`tools::js_repl::tests::js_repl_imported_local_files_can_access_repl_globals`:
dotslash download hit `mktemp: mkdtemp failed ... Operation not
permitted` in the sandbox temp dir.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/session/turn.rs

@@ -78,6 +78,7 @@ use codex_protocol::items::UserMessageItem;
 use codex_protocol::items::build_hook_prompt_message;
 use codex_protocol::models::BaseInstructions;
 use codex_protocol::models::ContentItem;
+use codex_protocol::models::MessagePhase;
 use codex_protocol::models::ResponseInputItem;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::protocol::AgentMessageContentDeltaEvent;
@@ -1944,6 +1945,25 @@ async fn try_run_sampling_request(
                     cancellation_token: cancellation_token.child_token(),
                 };
 
+                let preempt_for_mailbox_mail = match &item {
+                    ResponseItem::Message { role, phase, .. } => {
+                        role == "assistant" && matches!(phase, Some(MessagePhase::Commentary))
+                    }
+                    ResponseItem::Reasoning { .. } => true,
+                    ResponseItem::LocalShellCall { .. }
+                    | ResponseItem::FunctionCall { .. }
+                    | ResponseItem::ToolSearchCall { .. }
+                    | ResponseItem::FunctionCallOutput { .. }
+                    | ResponseItem::CustomToolCall { .. }
+                    | ResponseItem::CustomToolCallOutput { .. }
+                    | ResponseItem::ToolSearchOutput { .. }
+                    | ResponseItem::WebSearchCall { .. }
+                    | ResponseItem::ImageGenerationCall { .. }
+                    | ResponseItem::GhostSnapshot { .. }
+                    | ResponseItem::Compaction { .. }
+                    | ResponseItem::Other => false,
+                };
+
                 let output_result =
                     match handle_output_item_done(&mut ctx, item, previously_active_item)
                         .instrument(handle_responses)
@@ -1959,6 +1979,13 @@ async fn try_run_sampling_request(
                     last_agent_message = Some(agent_message);
                 }
                 needs_follow_up |= output_result.needs_follow_up;
+                // todo: remove before stabilizing multi-agent v2
+                if preempt_for_mailbox_mail && sess.mailbox_rx.lock().await.has_pending() {
+                    break Ok(SamplingRequestResult {
+                        needs_follow_up: true,
+                        last_agent_message,
+                    });
+                }
             }
             ResponseEvent::OutputItemAdded(item) => {
                 if let ResponseItem::CustomToolCall { call_id, name, .. } = &item {