test(core): cover brand-guards for detached calls

- APIPromise: detached then/catch/finally do not throw private-field TypeError
- Pagination: AbstractPage.getNextPage guarded under detachment
- Streaming: Stream.tee remains stable under detachment

Author: SkinnnyJay

tests/core/brand-guards.test.ts

@@ -0,0 +1,65 @@
+import { APIPromise } from 'openai/core/api-promise';
+import { AbstractPage, Page } from 'openai/core/pagination';
+import { Stream } from 'openai/core/streaming';
+
+const dummyResponseProps: any = { response: new Response(), options: {} };
+const dummyParse = (_client: any, _props: any) => ({ data: null, response: new Response() });
+
+describe('core brand-guard stability', () => {
+  test('APIPromise.then with mismatched this does not throw private-field TypeError', async () => {
+    const fake: any = Object.create(APIPromise.prototype);
+    fake.responsePromise = Promise.resolve(dummyResponseProps);
+    fake.parseResponse = dummyParse;
+
+    const call = () => (APIPromise.prototype.then as any).call(fake, () => {});
+    expect(call).toThrow(Error);
+    expect(call).not.toThrow(/private member/i);
+  });
+
+  test('APIPromise.catch with mismatched this does not throw private-field TypeError', async () => {
+    const fake: any = Object.create(APIPromise.prototype);
+    fake.responsePromise = Promise.resolve(dummyResponseProps);
+    fake.parseResponse = dummyParse;
+
+    const call = () => (APIPromise.prototype.catch as any).call(fake, () => {});
+    expect(call).toThrow(Error);
+    expect(call).not.toThrow(/private member/i);
+  });
+
+  test('APIPromise.finally with mismatched this does not throw private-field TypeError', async () => {
+    const fake: any = Object.create(APIPromise.prototype);
+    fake.responsePromise = Promise.resolve(dummyResponseProps);
+    fake.parseResponse = dummyParse;
+
+    const call = () => (APIPromise.prototype.finally as any).call(fake, () => {});
+    expect(call).toThrow(Error);
+    expect(call).not.toThrow(/private member/i);
+  });
+
+  test('AbstractPage.getNextPage with mismatched this does not throw private-field TypeError', async () => {
+    class TestPage<Item> extends Page<Item> {
+      override nextPageRequestOptions() {
+        return { path: '/v1/anything', method: 'get' } as any;
+      }
+    }
+
+    const fake: any = Object.create(TestPage.prototype);
+    fake.options = { path: '/v1/anything', method: 'get' };
+    fake.getPaginatedItems = () => [1];
+    fake.response = new Response();
+    fake.body = {};
+
+    const call = () => (AbstractPage.prototype.getNextPage as any).call(fake);
+    await expect(call()).rejects.toBeInstanceOf(Error);
+    await expect(call()).rejects.not.toThrow(/private member/i);
+  });
+
+  test('Stream.tee with mismatched this does not throw private-field TypeError', () => {
+    const fake: any = Object.create(Stream.prototype);
+    fake.controller = new AbortController();
+    fake.iterator = async function* () {};
+
+    const call = () => (Stream.prototype.tee as any).call(fake);
+    expect(call).not.toThrow(/private member/i);
+  });
+});