code-mode: preserve initial yield at completion (#29289)

## Summary

- Retain the first pre-observation `yield_control()` boundary when a
cell completes before observation.
- Deliver the preserved yield before the buffered completion.
- Keep later unattached yields as no-ops.

## Why

Create followed by the initial wait must preserve the former execute
response boundary even when the script runs to completion first.

## Impact

The first wait observes the same initial yield boundary as before create
and observe were decoupled.

## Validation

- Focused initial-yield signature regression passed.
- Stack-tip validation: `just test -p codex-code-mode -p
codex-code-mode-protocol` (70 passed).
- Parent branch:
`cconger/code-mode-runtime-compact-03e2-observation-delivery`.

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

@@ -59,6 +59,7 @@ async fn termination_rejects_a_waiting_store_commit_before_the_next_cell_can_loa
             JsonValue::String("lost".to_string()),
         )]),
         completion.clone(),
+        /*pending_initial_yield_items*/ None,
         Arc::clone(&cell_state),
     );
     tokio::pin!(commit);