Include the final agent message in turn completion summaries (#34777)

## Why

Streamed message deltas can be dropped when the transport is saturated, leaving
clients with a truncated final response.

## What changed

- Include the last non-empty final agent message in successful `turn/completed`
  notifications and mark the items as a summary.
- Treat completed message content as authoritative in the TUI, repairing an
  incomplete stream while avoiding duplicate messages when `item/completed` was
  already received.
- Continue backfilling full turn items for persisted `codex exec` threads when a
  completion contains only the summary view.

## Testing

Added coverage for completion summaries, dropped stream deltas, duplicate item
handling, and directive-only responses.

GitOrigin-RevId: ffe9d36d683d6b6690e4a468ff68aa9189fe6ab7

codex-rs/exec/src/lib.rs

@@ -1397,7 +1397,7 @@ fn should_backfill_turn_completed_items(
         return false;
     };
 
-    !thread_ephemeral && payload.turn.items.is_empty()
+    !thread_ephemeral && payload.turn.items_view != codex_app_server_protocol::TurnItemsView::Full
 }
 
 fn turn_items_for_thread(

codex-rs/exec/src/lib_tests.rs

@@ -396,13 +396,13 @@ fn turn_items_for_thread_returns_matching_turn_items() {
 }
 
 #[test]
-fn should_backfill_turn_completed_items_skips_ephemeral_threads() {
+fn should_backfill_turn_completed_items_backfills_persisted_summaries_only() {
     let notification =
         ServerNotification::TurnCompleted(codex_app_server_protocol::TurnCompletedNotification {
             thread_id: "thread-1".to_string(),
             turn: codex_app_server_protocol::Turn {
                 id: "turn-1".to_string(),
-                items_view: codex_app_server_protocol::TurnItemsView::Full,
+                items_view: codex_app_server_protocol::TurnItemsView::Summary,
                 items: Vec::new(),
                 status: codex_app_server_protocol::TurnStatus::Completed,
                 error: None,
@@ -416,6 +416,10 @@ fn should_backfill_turn_completed_items_skips_ephemeral_threads() {
         /*thread_ephemeral*/ true,
         &notification
     ));
+    assert!(should_backfill_turn_completed_items(
+        /*thread_ephemeral*/ false,
+        &notification
+    ));
 }
 
 #[test]