Improve Gemini CLI provider retries and headers (#670)

Improve Gemini CLI provider retries and headers

- Add Antigravity endpoint fallback (tries daily sandbox then prod when baseUrl is unset)
- Parse retry delays from headers (Retry-After, x-ratelimit-reset, x-ratelimit-reset-after) before body parsing
- Derive stable sessionId from first user message for cache affinity
- Retry empty SSE streams with backoff without duplicate start/done events
- Add anthropic-beta header for Claude thinking models only

packages/ai/test/google-gemini-cli-empty-stream.test.ts

@@ -0,0 +1,108 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { streamGoogleGeminiCli } from "../src/providers/google-gemini-cli.js";
+import type { Context, Model } from "../src/types.js";
+
+const originalFetch = global.fetch;
+
+afterEach(() => {
+	global.fetch = originalFetch;
+	vi.restoreAllMocks();
+});
+
+describe("google-gemini-cli empty stream retry", () => {
+	it("retries empty SSE responses without duplicate start", async () => {
+		const emptyStream = new ReadableStream<Uint8Array>({
+			start(controller) {
+				controller.close();
+			},
+		});
+
+		const sse = `${[
+			`data: ${JSON.stringify({
+				response: {
+					candidates: [
+						{
+							content: { role: "model", parts: [{ text: "Hello" }] },
+							finishReason: "STOP",
+						},
+					],
+					usageMetadata: {
+						promptTokenCount: 1,
+						candidatesTokenCount: 1,
+						totalTokenCount: 2,
+					},
+				},
+			})}`,
+		].join("\n\n")}\n\n`;
+
+		const encoder = new TextEncoder();
+		const dataStream = new ReadableStream<Uint8Array>({
+			start(controller) {
+				controller.enqueue(encoder.encode(sse));
+				controller.close();
+			},
+		});
+
+		let callCount = 0;
+		const fetchMock = vi.fn(async () => {
+			callCount += 1;
+			if (callCount === 1) {
+				return new Response(emptyStream, {
+					status: 200,
+					headers: { "content-type": "text/event-stream" },
+				});
+			}
+			return new Response(dataStream, {
+				status: 200,
+				headers: { "content-type": "text/event-stream" },
+			});
+		});
+
+		global.fetch = fetchMock as typeof fetch;
+
+		const model: Model<"google-gemini-cli"> = {
+			id: "gemini-2.5-flash",
+			name: "Gemini 2.5 Flash",
+			api: "google-gemini-cli",
+			provider: "google-gemini-cli",
+			baseUrl: "https://cloudcode-pa.googleapis.com",
+			reasoning: false,
+			input: ["text"],
+			cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+			contextWindow: 128000,
+			maxTokens: 8192,
+		};
+
+		const context: Context = {
+			messages: [{ role: "user", content: "Say hello", timestamp: Date.now() }],
+		};
+
+		const stream = streamGoogleGeminiCli(model, context, {
+			apiKey: JSON.stringify({ token: "token", projectId: "project" }),
+		});
+
+		let startCount = 0;
+		let doneCount = 0;
+		let text = "";
+
+		for await (const event of stream) {
+			if (event.type === "start") {
+				startCount += 1;
+			}
+			if (event.type === "done") {
+				doneCount += 1;
+			}
+			if (event.type === "text_delta") {
+				text += event.delta;
+			}
+		}
+
+		const result = await stream.result();
+
+		expect(text).toBe("Hello");
+		expect(result.stopReason).toBe("stop");
+		expect(startCount).toBe(1);
+		expect(doneCount).toBe(1);
+		expect(fetchMock).toHaveBeenCalledTimes(2);
+	});
+});