[Flight] Gated test for dropped transport of undefined object values (#26478)

## Summary

With #26349 we now serialize
`undefined`. However, deserializing it on the client is currently
indistinguishable from the value missing entirely due to how
`JSON.parse` treats `undefined` return value of reviver functions.

This leads to inconsistent behavior of the `Object.hasOwn` or `in`
operator (used for narrowing in TypeScript). In TypeScript-speak, `{
prop: T | undefined}` will arrive as `{ prop?: T }`.

## How did you test this change?

- Added test that is expected to fail. Though ideally the implementation
of the component would not care whether it's used on the client or
server.

Author: eps1lon

packages/react-client/src/__tests__/ReactFlight-test.js

@@ -213,6 +213,40 @@ describe('ReactFlight', () => {
     expect(ReactNoop).toMatchRenderedOutput(null);
   });
 
+  // @gate FIXME
+  it('should transport undefined object values', async () => {
+    function ServerComponent(props) {
+      return 'prop' in props
+        ? `\`prop\` in props as '${props.prop}'`
+        : '`prop` not in props';
+    }
+    const ClientComponent = clientReference(ServerComponent);
+
+    const model = (
+      <>
+        <div>
+          Server: <ServerComponent prop={undefined} />
+        </div>
+        <div>
+          Client: <ClientComponent prop={undefined} />
+        </div>
+      </>
+    );
+
+    const transport = ReactNoopFlightServer.render(model);
+
+    await act(async () => {
+      ReactNoop.render(await ReactNoopFlightClient.read(transport));
+    });
+
+    expect(ReactNoop).toMatchRenderedOutput(
+      <>
+        <div>Server: `prop` in props as 'undefined'</div>
+        <div>Client: `prop` in props as 'undefined'</div>
+      </>,
+    );
+  });
+
   it('can render an empty fragment', async () => {
     function Empty() {
       return <React.Fragment />;