Fix explicit turn start index test

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

codex-rs/core/src/thread_manager_tests.rs

@@ -544,7 +544,7 @@ async fn interrupted_fork_snapshot_preserves_explicit_turn_id() {
         SnapshotTurnState {
             ends_mid_turn: true,
             active_turn_id: Some("turn-explicit".to_string()),
-            active_turn_start_index: Some(0),
+            active_turn_start_index: Some(1),
         },
     );