Make gRPC code-mode yield tests deterministic (#38321)

## What changed

- Use a never-resolving promise when verifying that a session continues to
  enforce its yield limit after terminating a cell.
- Use `yield_control()` to create the yielded cell in the notification-draining
  test instead of relying on timer scheduling.

GitOrigin-RevId: 01cdaa94e5b8b5c89c6fa98c4bbdb28a690678e0

codex-rs/code-mode-host/tests/grpc.rs

@@ -877,8 +877,11 @@ async fn sessions_enforce_independent_yield_limits() -> Result<()> {
         })
     );
     assert_eq!(
-        execute(&limited, request(r#"text("recovered");"#)).await?,
-        text_response("2", "recovered")
+        execute(&limited, request("await new Promise(() => {});")).await?,
+        RuntimeResponse::Yielded {
+            cell_id: cell_id("2"),
+            content_items: Vec::new(),
+        }
     );
     limited.shutdown().await.map_err(anyhow::Error::msg)?;
     other.shutdown().await.map_err(anyhow::Error::msg)?;

codex-rs/code-mode-host/tests/grpc_notifications.rs

@@ -183,10 +183,7 @@ async fn completed_waits_drain_pending_notifications_before_returning() -> Resul
         .create_session(delegate.clone())
         .await
         .map_err(anyhow::Error::msg)?;
-    let mut pending = request(
-        r#"await new Promise(resolve => setTimeout(resolve, 25)); notify("notice"); text("done");"#,
-    );
-    pending.yield_time_ms = Some(/*value*/ 1);
+    let pending = request(r#"yield_control(); notify("notice"); text("done");"#);
     let cell = session.execute(pending).await.map_err(anyhow::Error::msg)?;
     assert_eq!(
         cell.initial_response().await.map_err(anyhow::Error::msg)?,