[regression] Fix ephemeral turn backfill in exec (#16795)

Addresses #16781

Problem: `codex exec --ephemeral` backfilled empty `turn/completed`
items with `thread/read(includeTurns=true)`, which app-server rejects
for ephemeral threads.

This is a regression introduced in the recent conversion of "exec" to
use app server rather than call the core directly.

Solution: Skip turn-item backfill for ephemeral exec threads while
preserving the existing recovery path for non-ephemeral sessions.

codex-rs/exec/src/lib_tests.rs

@@ -293,6 +293,25 @@ fn turn_items_for_thread_returns_matching_turn_items() {
     assert_eq!(turn_items_for_thread(&thread, "missing-turn"), None);
 }
 
+#[test]
+fn should_backfill_turn_completed_items_skips_ephemeral_threads() {
+    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: Vec::new(),
+                status: codex_app_server_protocol::TurnStatus::Completed,
+                error: None,
+            },
+        });
+
+    assert!(!should_backfill_turn_completed_items(
+        /*thread_ephemeral*/ true,
+        &notification
+    ));
+}
+
 #[test]
 fn canceled_mcp_server_elicitation_response_uses_cancel_action() {
     let value = canceled_mcp_server_elicitation_response()