fix(ai): kept antigravity failover before advisor silence

Gated empty-STOP silence acceptance on the last Cloud Code Assist endpoint so an earlier endpoint returning only empty streams still fails over instead of being recorded as a valid silent review.

Added an Antigravity auto-mode regression covering failover exhaustion before silence.

packages/ai/src/providers/google-gemini-cli.ts

@@ -1006,7 +1006,12 @@ export const streamGoogleGeminiCli: StreamFunction<"google-gemini-cli"> = (
 						}
 
 						const streamed = await streamResponse(currentResponse);
-						const acceptedSilence = options?.acceptEmptyResponse === true && !streamed.strippedPlanningLeak;
+						// Only accept an empty STOP as valid silence once every fallback
+						// endpoint is exhausted: an earlier endpoint returning empty
+						// successful streams must still fail over (Antigravity auto mode)
+						// rather than be recorded as a real silent review.
+						const acceptedSilence =
+							options?.acceptEmptyResponse === true && !streamed.strippedPlanningLeak && isLastEndpoint;
 						if (output.stopReason !== "stop" || streamed.meaningful || acceptedSilence) {
 							receivedContent = streamed.meaningful || acceptedSilence;
 							break;

packages/ai/test/google-empty-response-retry.test.ts

@@ -364,6 +364,34 @@ describe("Google empty-response retry (Cloud Code Assist path)", () => {
 		expect(textOf(result)).toBe("Recovered.");
 	});
 
+	it("exhausts Antigravity auto failover before accepting silence", async () => {
+		const requestedEndpoints: string[] = [];
+		const fetchMock: FetchImpl = async input => {
+			const endpoint = endpointFromInput(input);
+			requestedEndpoints.push(endpoint);
+			return withResponseUrl(sse(ccaChunk("")), endpoint);
+		};
+
+		const stream = streamGoogleGeminiCli(antigravityModel, context, {
+			apiKey: JSON.stringify({ token: "token", projectId: "proj-123" }),
+			antigravityEndpointMode: "auto",
+			acceptEmptyResponse: true,
+			fetch: fetchMock,
+		});
+		const result = await stream.result();
+
+		// Daily still burns its empty-response budget and fails over; only the
+		// last (sandbox) endpoint records the empty STOP as valid silence.
+		expect(requestedEndpoints).toEqual([
+			ANTIGRAVITY_DAILY_ENDPOINT,
+			ANTIGRAVITY_DAILY_ENDPOINT,
+			ANTIGRAVITY_DAILY_ENDPOINT,
+			ANTIGRAVITY_SANDBOX_ENDPOINT,
+		]);
+		expect(result.stopReason).toBe("stop");
+		expect(result.errorMessage).toBeUndefined();
+	});
+
 	for (const { mode, endpoint } of [
 		{ mode: "production", endpoint: ANTIGRAVITY_DAILY_ENDPOINT },
 		{ mode: "sandbox", endpoint: ANTIGRAVITY_SANDBOX_ENDPOINT },