Allow returning a temporary reference inside an async function (facebook#33761)

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## Summary

Fixes `await`-ing and returning temporary references in `async`
functions. These two operations invoke `.then()` under the hood if it is
available, which currently results in an "Cannot access then on the
server. You cannot dot into a temporary client reference..." error. This
can easily be reproduced by returning a temporary reference from a
server function.

Fixes facebook#33534 

## How did you test this change?
I added a test in a new test file. I wasn't sure where else to put it.
<img width="771" height="138" alt="image"
src="https://github.com/user-attachments/assets/09ffe6eb-271a-4842-a9fe-c68e17b3fb41"
/>


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Author: denk0403

packages/react-server/src/ReactFlightServerTemporaryReferences.js

@@ -70,6 +70,9 @@ const proxyHandlers = {
             `Instead, you can export a Client Component wrapper ` +
             `that itself renders a Client Context Provider.`,
         );
+      // Allow returning a temporary reference from an async function
+      case 'then':
+        return undefined;
     }
     throw new Error(
       // eslint-disable-next-line react-internal/safe-string-coercion

packages/react-server/src/__tests__/ReactFlightServerTemporaryReferences-test.js

@@ -0,0 +1,36 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ * @jest-environment node
+ */
+
+'use strict';
+
+let ReactFlightServerTemporaryReferences;
+
+describe('ReactFlightServerTemporaryReferences', () => {
+  beforeEach(() => {
+    jest.resetModules();
+    ReactFlightServerTemporaryReferences = require('react-server/src/ReactFlightServerTemporaryReferences');
+  });
+
+  it('can return a temporary reference from an async function', async () => {
+    const temporaryReferenceSet =
+      ReactFlightServerTemporaryReferences.createTemporaryReferenceSet();
+    const temporaryReference =
+      ReactFlightServerTemporaryReferences.createTemporaryReference(
+        temporaryReferenceSet,
+        'test',
+      );
+
+    async function foo() {
+      return temporaryReference;
+    }
+
+    await expect(foo()).resolves.toBe(temporaryReference);
+  });
+});