Bug: [Flight] Async server components in `ai/rsc` not rendered correctly

[unstubbable]

When returning an async server component from an ai/rsc tool's render function, a reference to it is not serialized as a lazy node, even though it might not have been emitted as a row yet. Currently the React Flight Client is not resilient to this. This leads to the reference not being resolved correctly, and then null is rendered instead of the element.

The bug was introduced with #28283.

React version: 18.3.0-canary-6c3b8dbfe-20240226 (via Next.js v14.2.0-canary.30)

Steps To Reproduce

1. Build ai/rsc example according to https://sdk.vercel.ai/docs/concepts/ai-rsc#build-your-app
2. Move data fetching from render function to an async server component

(It's probably also reproducible by writing a unit test that replicates what's being done in https://github.com/vercel/ai/blob/main/packages/core/rsc/utils.tsx#L17-L50. I might try this later... done)

Link to code example: unstubbable/ai-rsc-test#1
This contains the code of step 2 from above, has more details about the expected and actual RSC responses, and also shows in the comments how I arrived at my conclusion.

[unstubbable]

A potential fix is mentioned in the code comments of #28283:

We should really detect whether this already was emitted and synchronously available. In that case we can refer to it synchronously and only make it lazy otherwise. We currently don't have a data structure that lets us see that though.