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/mod.rs

@@ -256,6 +256,7 @@ impl<D: SessionRuntimeDelegate> CellHost for RuntimeCellHost<D> {
         &self,
         stored_value_writes: HashMap<String, JsonValue>,
         event: CellEvent,
+        pending_initial_yield_items: Option<Vec<OutputItem>>,
         cell_state: Arc<CellState>,
     ) -> CompletionCommit {
         let cancellation_token = cell_state.cancellation_token();
@@ -266,7 +267,9 @@ impl<D: SessionRuntimeDelegate> CellHost for RuntimeCellHost<D> {
             }
             stored_values = self.inner.stored_values.lock() => stored_values,
         };
-        cell_state.commit_completion(event, || stored_values.extend(stored_value_writes))
+        cell_state.commit_completion(event, pending_initial_yield_items, || {
+            stored_values.extend(stored_value_writes);
+        })
     }
 
     async fn closed(&self) {