Simplify code-mode cell completion delivery (#57229)

## What changed

Remove unused pending initial-yield state and its plumbing from cell completion handling. Route completion delivery through `send_cell_event`, retaining the completion event and original timestamp when an observer has disconnected.

## Testing

Extend `failed_completion_delivery_rebuffers_the_event` to verify that failed delivery attempts preserve output and report the full elapsed buffering duration on a later successful observation.

GitOrigin-RevId: eebe990a4f8258f1ccb12d0694945678f0cc3a8f

codex-rs/code-mode-runtime/src/session_runtime/tests.rs

@@ -119,7 +119,6 @@ async fn termination_rejects_a_waiting_store_commit_before_the_next_cell_can_loa
             Arc::new(JsonValue::String("lost".to_string())),
         )]),
         completion.clone(),
-        /*pending_initial_yield_items*/ None,
         Arc::clone(&cell_state),
     );
     tokio::pin!(commit);