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-claude-thinking-header.test.ts

@@ -0,0 +1,103 @@
+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;
+const apiKey = JSON.stringify({ token: "token", projectId: "project" });
+
+const createSseResponse = () => {
+	const sse = `${[
+		`data: ${JSON.stringify({
+			response: {
+				candidates: [
+					{
+						content: { role: "model", parts: [{ text: "Hello" }] },
+						finishReason: "STOP",
+					},
+				],
+			},
+		})}`,
+	].join("\n\n")}\n\n`;
+
+	const encoder = new TextEncoder();
+	const stream = new ReadableStream<Uint8Array>({
+		start(controller) {
+			controller.enqueue(encoder.encode(sse));
+			controller.close();
+		},
+	});
+
+	return new Response(stream, {
+		status: 200,
+		headers: { "content-type": "text/event-stream" },
+	});
+};
+
+afterEach(() => {
+	global.fetch = originalFetch;
+	vi.restoreAllMocks();
+});
+
+describe("google-gemini-cli Claude thinking header", () => {
+	const context: Context = {
+		messages: [{ role: "user", content: "Say hello", timestamp: Date.now() }],
+	};
+
+	it("adds anthropic-beta for Claude thinking models", async () => {
+		const fetchMock = vi.fn(async (_input: string | URL, init?: RequestInit) => {
+			const headers = new Headers(init?.headers);
+			expect(headers.get("anthropic-beta")).toBe("interleaved-thinking-2025-05-14");
+			return createSseResponse();
+		});
+
+		global.fetch = fetchMock as typeof fetch;
+
+		const model: Model<"google-gemini-cli"> = {
+			id: "claude-opus-4-5-thinking",
+			name: "Claude Opus 4.5 Thinking",
+			api: "google-gemini-cli",
+			provider: "google-antigravity",
+			baseUrl: "https://cloudcode-pa.googleapis.com",
+			reasoning: true,
+			input: ["text"],
+			cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+			contextWindow: 128000,
+			maxTokens: 8192,
+		};
+
+		const stream = streamGoogleGeminiCli(model, context, { apiKey });
+		for await (const _event of stream) {
+			// exhaust stream
+		}
+		await stream.result();
+	});
+
+	it("does not add anthropic-beta for Gemini models", async () => {
+		const fetchMock = vi.fn(async (_input: string | URL, init?: RequestInit) => {
+			const headers = new Headers(init?.headers);
+			expect(headers.has("anthropic-beta")).toBe(false);
+			return createSseResponse();
+		});
+
+		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 stream = streamGoogleGeminiCli(model, context, { apiKey });
+		for await (const _event of stream) {
+			// exhaust stream
+		}
+		await stream.result();
+	});
+});