fix(ai): restore FakeOpenAI mock .withResponse() so openai-completions tests pass

The openai-completions provider now calls .withResponse() on the
create() result to surface HTTP status/headers for the new onResponse
hook (d131fcd4). The FakeOpenAI test mock only returned an async
iterable, so 4 tests in openai-completions-tool-choice.test.ts that
actually consume the stream failed with "withResponse is not a function".

Updated the mock to return a Promise augmented with .withResponse()
resolving to { data: <async iterable>, response: { status, headers } }.

closes #3304

packages/ai/test/openai-completions-tool-choice.test.ts

@@ -24,9 +24,9 @@ vi.mock("openai", () => {
 	class FakeOpenAI {
 		chat = {
 			completions: {
-				create: async (params: unknown) => {
+				create: (params: unknown) => {
 					mockState.lastParams = params;
-					return {
+					const stream = {
 						async *[Symbol.asyncIterator]() {
 							const chunks = mockState.chunks ?? [
 								{
@@ -44,6 +44,17 @@ vi.mock("openai", () => {
 							}
 						},
 					};
+					const promise = Promise.resolve(stream) as Promise<typeof stream> & {
+						withResponse: () => Promise<{
+							data: typeof stream;
+							response: { status: number; headers: Headers };
+						}>;
+					};
+					promise.withResponse = async () => ({
+						data: stream,
+						response: { status: 200, headers: new Headers() },
+					});
+					return promise;
 				},
 			},
 		};